Fix pip-sync --python-executable evaluating markers for the wrong environment

[gschaffner]

Proposed patchset that fixes #2115.

I've marked this as a draft PR because it's just a fix; it does not yet include an automated test. The test for this seems difficult to fit into the current testing framework—the test requires access to multiple (differing) interpreters—so I was hoping someone with more familiarity could help with that.

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[gschaffner]

I do not currently understand the failures of test_python_executable_option (assert 0 == 2) on various piplatest on GHA. tox -r -e "$(echo py{38,39,310,311,312,py3}-pip{previous,latest,main}-coverage | tr ' ' ',')" -- --no-cov -n=0 -v -k test_python_executable_option passes locally.

[chrysle]

The test for this seems difficult to fit into the current testing framework—the test requires access to multiple (differing) interpreters—so I was hoping someone with more familiarity could help with that.

Basically, we might just as well use the same interpreter and a corresponding marker?

[gschaffner]

Basically, we might just as well use the same interpreter and a corresponding marker?

I don't think I understand, where are you suggesting putting a marker?

My understanding is that for the bug to occur, InstallRequirement.markers.evaluate(environment) needs to produce a different result with the pip-sync interpreter's environment and with the --python-executable=... interpreter's environment. AFAIK this is only possible if the two environments differ, and AFAIK that is only possible if they are not the same interpreter.

[gschaffner]

Applied review suggestions and fixed a crash on Windows.