… add (#11404)

* fix(global): avoid doubled modulesDir when approving builds in global add

The global add → approve-builds flow used to forward an absolute
`modulesDir` (`<installDir>/node_modules`) into the install run by
`approve-builds`. The install layer treats `modulesDir` as a path
relative to `lockfileDir` and joins it again — producing a doubled
path on Windows because `path.join` does not collapse an embedded
absolute path. The hoist step then failed with `ENOENT` while trying
to symlink under `<installDir>\<installDir>\node_modules\.pnpm\...`.

Closes #11403.

* test: type test fixtures correctly

* fix(install): tolerate absolute modulesDir in headless install context

Replace the prior unit test (which only checked the call shape) with an
integration test that exercises `install()` with an absolute `modulesDir`
through both the regular and frozen-lockfile paths — the failure mode the
global add → approve-builds chain originally hit on Windows.

`headlessInstall` and `readProjectsContext` now resolve `modulesDir` via
`pathAbsolute` instead of `path.join(lockfileDir, modulesDir)`, so an
absolute value no longer produces a doubled prefix. The
`promptApproveGlobalBuilds` change from the previous commit is retained
as the contract-level fix.

* test: add e2e test driving the pnpm CLI with --modules-dir=<abs>

Replace the programmatic install() regression test with an e2e test in
pnpm/test/install/absoluteModulesDir.ts that runs the bundled pnpm
binary with `pnpm install --modules-dir=<abs>` (regular and frozen).
This is the closest CLI-level reproduction of the doubled-prefix path
bug from #11403 — the bug fired specifically in the headless install
path that --frozen-lockfile triggers.

* test(global): drive add -g + approve-builds chain end-to-end

Add an e2e test that runs the bundled pnpm CLI through the full
`pnpm add -g <pkg-with-build>` → approve-builds → install chain that
produced the doubled-prefix `ENOENT` in #11403.

The chain only fires when `process.stdin.isTTY` is true, which CI
subprocesses don't satisfy. Add a test-only env var
`PNPM_AUTO_APPROVE_BUILDS_FOR_TESTS` that bypasses the TTY guard in
`promptApproveGlobalBuilds` and forwards `all: true` so `approve-builds`
skips its multiselect and confirm prompts. The post-approval install
then runs the same code path a real user hit, and the test asserts the
build artifacts ended up in the global install dir.

Replaces the narrower `--modules-dir=<abs>` regression test, which
only exercised the install layer and not the global-add flow that
originally surfaced the bug.

* test: enable global add -g + approve-builds e2e test on Windows

- Switch to @pnpm.e2e/install-script-example which is cross-platform.
- Use pathAbsolute for modulesDir to prevent doubled path bugs on Windows.
- Add path-absolute dependency to affected packages.