Improve pip config set error when PIP_CONFIG_FILE is non-regular

[gcomneno]

Fixes #11880.

When PIP_CONFIG_FILE points to a non-regular file (e.g. /dev/null), pip config set
previously failed with "Fatal Internal error [id=2]". This change reports a clear,
actionable ConfigurationError instead, and adds a functional test covering the case.

Tests:

• nox -s test-3.12 -- tests/functional/test_configuration.py

[gcomneno]

Repro: PIP_CONFIG_FILE=/dev/null pip config set ... previously produced “Fatal Internal error [id=2]”.
This PR turns that into a clear ConfigurationError and adds functional coverage.

Tests: nox -s test-3.12 -- tests/functional/test_configuration.py

[gcomneno]

Minor wording tweak to align with pip-style error messages.

[pfmoore]

Can we not make the test work on windows using the equivalent windows special file (NUL)?

[pradyunsg]

https://docs.python.org/3/library/os.html#os.devnull might be relevant here.

[gcomneno]

I tried making the test cross-platform: instead of skipping Windows due to hardcoded /dev/null, use os.devnull ("/dev/null" on POSIX, "nul" on Windows) and drop the skip.

Patch is ready on my fork: commit a58ede1 (branch pr-13790).

To apply:

git fetch https://github.com/gcomneno/pip.git pr-13790
git cherry-pick a58ede1fc

Rationale: avoids hardcoding /dev/null and the Windows skip while keeping the intent of the test (non-regular PIP_CONFIG_FILE -> human-friendly error).

[gcomneno]

Quick follow-up: I have a small commit that makes the functional test cross-platform by using os.devnull instead of hardcoding /dev/null (commit a58ede1). Happy to push it directly onto this PR if you prefer.

[gcomneno]

I pushed the cross-platform test tweak (use os.devnull instead of hardcoding /dev/null) onto the PR branch, so no cherry-pick is needed.