Describe the bug

TomlConfigParser fails to handle environment variables whose value is a JSON-encoded string.

To Reproduce
How can we reproduce the problem? Please be specific. Don't link to a failing CI job. Answer the questions below:

1. What version of Python are you using? 3.10.2
2. What version of coverage.py shows the problem? The output of coverage debug sys is helpful. 6.2
3. What versions of what packages do you have installed? The output of pip freeze is helpful.

tomli==2.0.0
pytest==6.2.5
pytest-cov==3.0.0

1. What code shows the problem? Give us a specific commit of a specific repo that we can check out. If you've already worked around the problem, please provide a commit before that fix.

See https://github.com/stevenhoelscher/coveragepy-repro for reproduction of the bug.

1. What commands did you run?

pytest

Expected behavior
When running pytest with pytest-cov (which relies on coveragepy under the hood), another tool's pyproject.toml should not affect coveragepy's ability to parse the configuration file.

Additional context

As you can see in this example, I try to add another tool's configuration to my pyproject.toml. But the substitute_variables function ends up interpolating the environment variable and invalidating the configuration prior to parsing:

'[[tool.docker.env_vars]]\nname = "foo"\nvalue = "\\"{"foo": "bar"}\\""\n\n[tool.pytest.ini_options]\ntestpaths = "src/tests"\naddopts = "--cov=src"\n\n[tool.coverage.run]\nomit = ["src/tests/*"]\n'

>>> import tomli
>>> s = '[[tool.docker.env_vars]]\nname = "foo"\nvalue = "\\"{"foo": "bar"}\\""\n\n[tool.pytest.ini_options]\ntestpaths = "src/tests"\naddopts = "--cov=src"\n\n[tool.coverage.run]\nomit = ["src/tests/*"]\n'
>>> tomli.loads(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/developer/code/coveragepy-repro/.venv/lib/python3.10/site-packages/tomli/_parser.py", line 116, in loads
    raise suffixed_err(
tomli.TOMLDecodeError: Expected newline or end of document after a statement (at line 3, column 14)