Section substitution doesn't work in `setenv`

## Issue

`setenv` variables are not available when running `commands` from a tox test environment, if the variables were defined using [Substitution for values from other sections](https://tox.wiki/en/4.14.2/config.html#substitution-for-values-from-other-sections).

That seems some kind of caching issue, as during some debugging I found that removing [this IF statement](https://github.com/tox-dev/tox/blob/main/src/tox/tox_env/api.py#L333-L334) makes `tox` work as I would expect.

I'm providing a small reproducible below.
That reproducible defines 3 environment variables through `setenv`: one directly, and 2 through substitution.
Only the variable defined directly is available when running `bash` from `commands`, while the ones defined through substitution are not available.

If we check the output of `tox config`, we can see all of the 3 variables correctly defined. And as I mentioned above, if I remove that `IF` statement, making it always update the variables on `environment_variables` property call, then all the 3 variables are available when running `bash`.

## Environment

Provide at least:

- OS: AlmaLinux 8.8

<details open>
<summary>Output of <code>pip list</code> of the host Python, where <code>tox</code> is installed</summary>

```console
$ pip list
Package       Version
------------- -------
cachetools    5.3.3
chardet       5.2.0
colorama      0.4.6
distlib       0.3.8
filelock      3.13.3
packaging     24.0
pip           24.0
platformdirs  4.2.0
pluggy        1.4.0
pyproject-api 1.6.1
setuptools    58.1.0
tomli         2.0.1
tox           4.14.2
virtualenv    20.25.1
```

</details>

## Output of running tox

<details open>
<summary>Output of <code>tox -rvv</code></summary>

```console
$ tox -rvv
py: 194 I find interpreter for spec PythonSpec(path=/var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/bin/python3.9) [virtualenv/discovery/builtin.py:58]
py: 194 D discover exe for PythonInfo(spec=CPython3.9.18.final.0-64, exe=/var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/bin/python3.9, platform=linux, version='3.9.18 (main, Feb 26 2024, 16:24:53) \n[GCC 8.5.0 20210514 (Red Hat 8.5.0-20)]', encoding_fs_io=utf-8-utf-8) in /var/lib/pgsql/.pyenv/versions/3.9.18 [virtualenv/discovery/py_info.py:438]
py: 195 D filesystem is case-sensitive [virtualenv/info.py:25]
py: 199 D got python info of %s from (PosixPath('/var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9'), PosixPath('/var/lib/pgsql/.local/share/virtualenv/py_info/1/7a41df77756a29a17ad174a1eadcfcb87b684555bf31d80d33de969f26bf0026.json')) [virtualenv/app_data/via_disk_folder.py:131]
py: 200 I proposed PythonInfo(spec=CPython3.9.18.final.0-64, system=/var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9, exe=/var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/bin/python3.9, platform=linux, version='3.9.18 (main, Feb 26 2024, 16:24:53) \n[GCC 8.5.0 20210514 (Red Hat 8.5.0-20)]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:65]
py: 201 D accepted PythonInfo(spec=CPython3.9.18.final.0-64, system=/var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9, exe=/var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/bin/python3.9, platform=linux, version='3.9.18 (main, Feb 26 2024, 16:24:53) \n[GCC 8.5.0 20210514 (Red Hat 8.5.0-20)]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:67]
py: 232 I create virtual environment via CPython3Posix(dest=/var/lib/pgsql/tox_issue/.tox/py, clear=False, no_vcs_ignore=False, global=False) [virtualenv/run/session.py:50]
py: 232 D create folder /var/lib/pgsql/tox_issue/.tox/py/bin [virtualenv/util/path/_sync.py:12]
py: 233 D create folder /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages [virtualenv/util/path/_sync.py:12]
py: 233 D write /var/lib/pgsql/tox_issue/.tox/py/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:33]
py: 234 D 	home = /var/lib/pgsql/.pyenv/versions/3.9.18/bin [virtualenv/create/pyenv_cfg.py:38]
py: 234 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:38]
py: 234 D 	version_info = 3.9.18.final.0 [virtualenv/create/pyenv_cfg.py:38]
py: 234 D 	virtualenv = 20.25.1 [virtualenv/create/pyenv_cfg.py:38]
py: 234 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:38]
py: 234 D 	base-prefix = /var/lib/pgsql/.pyenv/versions/3.9.18 [virtualenv/create/pyenv_cfg.py:38]
py: 235 D 	base-exec-prefix = /var/lib/pgsql/.pyenv/versions/3.9.18 [virtualenv/create/pyenv_cfg.py:38]
py: 235 D 	base-executable = /var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9 [virtualenv/create/pyenv_cfg.py:38]
py: 236 D symlink /var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9 to /var/lib/pgsql/tox_issue/.tox/py/bin/python [virtualenv/util/path/_sync.py:32]
py: 236 D create virtualenv import hook file /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/_virtualenv.pth [virtualenv/create/via_global_ref/api.py:91]
py: 237 D create /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/_virtualenv.py [virtualenv/create/via_global_ref/api.py:94]
py: 237 D ============================== target debug ============================== [virtualenv/run/session.py:52]
py: 237 D debug via /var/lib/pgsql/tox_issue/.tox/py/bin/python /var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/lib/python3.9/site-packages/virtualenv/create/debug.py [virtualenv/create/creator.py:200]
py: 237 D {
  "sys": {
    "executable": "/var/lib/pgsql/tox_issue/.tox/py/bin/python",
    "_base_executable": "/var/lib/pgsql/tox_issue/.tox/py/bin/python",
    "prefix": "/var/lib/pgsql/tox_issue/.tox/py",
    "base_prefix": "/var/lib/pgsql/.pyenv/versions/3.9.18",
    "real_prefix": null,
    "exec_prefix": "/var/lib/pgsql/tox_issue/.tox/py",
    "base_exec_prefix": "/var/lib/pgsql/.pyenv/versions/3.9.18",
    "path": [
      "/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python39.zip",
      "/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9",
      "/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/lib-dynload",
      "/var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages"
    ],
    "meta_path": [
      "<class '_virtualenv._Finder'>",
      "<class '_frozen_importlib.BuiltinImporter'>",
      "<class '_frozen_importlib.FrozenImporter'>",
      "<class '_frozen_importlib_external.PathFinder'>"
    ],
    "fs_encoding": "utf-8",
    "io_encoding": "utf-8"
  },
  "version": "3.9.18 (main, Feb 26 2024, 16:24:53) \n[GCC 8.5.0 20210514 (Red Hat 8.5.0-20)]",
  "makefile_filename": "/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/config-3.9-x86_64-linux-gnu/Makefile",
  "os": "<module 'os' from '/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/os.py'>",
  "site": "<module 'site' from '/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/site.py'>",
  "datetime": "<module 'datetime' from '/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/datetime.py'>",
  "math": "<module 'math' from '/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/lib-dynload/math.cpython-39-x86_64-linux-gnu.so'>",
  "json": "<module 'json' from '/var/lib/pgsql/.pyenv/versions/3.9.18/lib/python3.9/json/__init__.py'>"
} [virtualenv/run/session.py:53]
py: 272 I add seed packages via FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/var/lib/pgsql/.local/share/virtualenv) [virtualenv/run/session.py:57]
py: 280 D install pip from wheel /var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/pip-24.0-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:49]
py: 282 D install setuptools from wheel /var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/setuptools-69.1.0-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:49]
py: 283 D install wheel from wheel /var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/wheel-0.42.0-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:49]
py: 287 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-24.0-py3-none-any/pip to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/pip [virtualenv/util/path/_sync.py:40]
py: 289 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.42.0-py3-none-any/wheel to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/wheel [virtualenv/util/path/_sync.py:40]
py: 292 D copy /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/distutils-precedence.pth to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/distutils-precedence.pth [virtualenv/util/path/_sync.py:40]
py: 295 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/_distutils_hack to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/_distutils_hack [virtualenv/util/path/_sync.py:40]
py: 301 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/pkg_resources to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/pkg_resources [virtualenv/util/path/_sync.py:40]
py: 336 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.42.0-py3-none-any/wheel-0.42.0.dist-info to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/wheel-0.42.0.dist-info [virtualenv/util/path/_sync.py:40]
py: 348 D copy /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.42.0-py3-none-any/wheel-0.42.0.virtualenv to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/wheel-0.42.0.virtualenv [virtualenv/util/path/_sync.py:40]
py: 354 D generated console scripts wheel3 wheel3.9 wheel-3.9 wheel [virtualenv/seed/embed/via_app_data/pip_install/base.py:43]
py: 389 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/setuptools to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/setuptools [virtualenv/util/path/_sync.py:40]
py: 569 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/setuptools-69.1.0.dist-info to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/setuptools-69.1.0.dist-info [virtualenv/util/path/_sync.py:40]
py: 579 D copy /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-69.1.0-py3-none-any/setuptools-69.1.0.virtualenv to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/setuptools-69.1.0.virtualenv [virtualenv/util/path/_sync.py:40]
py: 580 D generated console scripts  [virtualenv/seed/embed/via_app_data/pip_install/base.py:43]
py: 725 D copy directory /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-24.0-py3-none-any/pip-24.0.dist-info to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/pip-24.0.dist-info [virtualenv/util/path/_sync.py:40]
py: 731 D copy /var/lib/pgsql/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-24.0-py3-none-any/pip-24.0.virtualenv to /var/lib/pgsql/tox_issue/.tox/py/lib/python3.9/site-packages/pip-24.0.virtualenv [virtualenv/util/path/_sync.py:40]
py: 733 D generated console scripts pip3.9 pip pip-3.9 pip3 [virtualenv/seed/embed/via_app_data/pip_install/base.py:43]
py: 734 I add activators for Bash, CShell, Fish, Nushell, PowerShell, Python [virtualenv/run/session.py:63]
py: 737 D write /var/lib/pgsql/tox_issue/.tox/py/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:33]
py: 737 D 	home = /var/lib/pgsql/.pyenv/versions/3.9.18/bin [virtualenv/create/pyenv_cfg.py:38]
py: 737 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:38]
py: 737 D 	version_info = 3.9.18.final.0 [virtualenv/create/pyenv_cfg.py:38]
py: 737 D 	virtualenv = 20.25.1 [virtualenv/create/pyenv_cfg.py:38]
py: 737 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:38]
py: 737 D 	base-prefix = /var/lib/pgsql/.pyenv/versions/3.9.18 [virtualenv/create/pyenv_cfg.py:38]
py: 738 D 	base-exec-prefix = /var/lib/pgsql/.pyenv/versions/3.9.18 [virtualenv/create/pyenv_cfg.py:38]
py: 738 D 	base-executable = /var/lib/pgsql/.pyenv/versions/3.9.18/bin/python3.9 [virtualenv/create/pyenv_cfg.py:38]
  py: OK (0.56 seconds)
  congratulations :) (0.64 seconds)
```

</details>

## Minimal example

* `tox.ini` content:

```ini
[tox]

[variables]
postgres_variant =
    pg: PG_VARIANT = PostgreSQL
    rds: PG_VARIANT = RDS

postgres_version =
    12: PG_MAJOR = 12
    13: PG_MAJOR = 13
    14: PG_MAJOR = 14
    15: PG_MAJOR = 15
    16: PG_MAJOR = 16

[testenv:{pg,rds}-{12,13,14,15,16}-test]
description = Issue reproducer

setenv =
    {[variables]postgres_variant}
    {[variables]postgres_version}
    CONTAINER_IMAGE = test:{env:PG_VARIANT}-{env:PG_MAJOR}

commands =
    bash -c "env | grep -P 'PG_(VARIANT|MAJOR)|CONTAINER_IMAGE'"

allowlist_externals =
    bash

```

* Output of `tox run`:

```console
$ tox run -e rds-13-test
rds-13-test: commands[0]> bash -c 'env | grep -P '"'"'PG_(VARIANT|MAJOR)|CONTAINER_IMAGE'"'"''
CONTAINER_IMAGE=test:RDS-13
  rds-13-test: OK (0.04=setup[0.03]+cmd[0.01] seconds)
  congratulations :) (0.11 seconds)
```

* Output of `tox config`:

```console
$ tox config -e rds-13-test
[testenv:rds-13-test]
type = VirtualEnvRunner
set_env =
  CONTAINER_IMAGE=test:RDS-13
  PG_MAJOR=13
  PG_VARIANT=RDS
  PIP_DISABLE_PIP_VERSION_CHECK=1
  PYTHONHASHSEED=3633014905
  PYTHONIOENCODING=utf-8
base = testenv
runner = virtualenv
description = Issue reproducer
depends =
env_name = rds-13-test
labels =
env_dir = /var/lib/pgsql/tox_issue/.tox/rds-13-test
env_tmp_dir = /var/lib/pgsql/tox_issue/.tox/rds-13-test/tmp
env_log_dir = /var/lib/pgsql/tox_issue/.tox/rds-13-test/log
suicide_timeout = 0.0
interrupt_timeout = 0.3
terminate_timeout = 0.2
platform =
pass_env =
  CC
  CCSHARED
  CFLAGS
  CPPFLAGS
  CURL_CA_BUNDLE
  CXX
  FORCE_COLOR
  HOME
  LANG
  LANGUAGE
  LDFLAGS
  LD_LIBRARY_PATH
  NO_COLOR
  PIP_*
  PKG_CONFIG
  PKG_CONFIG_PATH
  PKG_CONFIG_SYSROOT_DIR
  REQUESTS_CA_BUNDLE
  SSL_CERT_FILE
  TERM
  TMPDIR
  VIRTUALENV_*
  http_proxy
  https_proxy
  no_proxy
parallel_show_output = False
recreate = False
allowlist_externals = bash
pip_pre = False
install_command = python -I -m pip install '{packages}'
constrain_package_deps = False
use_frozen_constraints = False
list_dependencies_command = python -m pip freeze --all
commands_pre =
commands = bash -c 'env | grep -P '"'"'PG_(VARIANT|MAJOR)|CONTAINER_IMAGE'"'"''
commands_post =
change_dir = /var/lib/pgsql/tox_issue
args_are_paths = True
ignore_errors = False
ignore_outcome = False
base_python = /var/lib/pgsql/.pyenv/versions/3.9.18/envs/tox_issue/bin/python3.9
env_site_packages_dir = /var/lib/pgsql/tox_issue/.tox/rds-13-test/lib/python3.9/site-packages
env_bin_dir = /var/lib/pgsql/tox_issue/.tox/rds-13-test/bin
env_python = /var/lib/pgsql/tox_issue/.tox/rds-13-test/bin/python
py_dot_ver = 3.9
py_impl = cpython
deps =
system_site_packages = False
always_copy = False
download = False
skip_install = False
use_develop = False
package = skip
```

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Uh oh!

Section substitution doesn't work in `setenv` #3262

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Uh oh!

Section substitution doesn't work in setenv #3262

Description

Issue

Environment

Output of running tox

Minimal example

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Issue actions

Section substitution doesn't work in `setenv` #3262