Fix running python_sources with pex --executable

[cognifloyd]

This fixes the feature added in #20497 as it broke using pants run on a python_source if the file name has - or other invalid characters. Bug reported here:
https://pantsbuild.slack.com/archives/C046T6T9U/p1717624913138789?thread_ts=1717624913.138789&cid=C046T6T9U

The other integration test for the pex --executable feature only tested running pex_binary, not python_source. So, I missed applying some of the path logic to both cases.

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

❌ 2.20.x

I was unable to cherry-pick this PR to 2.20.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.20.x \
     && git checkout -b cherry-pick-21047-to-2.20.x FETCH_HEAD \
     && git cherry-pick 7970d308719df22b2cb483e950fc64b0315a4242

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21047" "2.20.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

❌ 2.21.x

I was unable to cherry-pick this PR to 2.21.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.21.x \
     && git checkout -b cherry-pick-21047-to-2.21.x FETCH_HEAD \
     && git cherry-pick 7970d308719df22b2cb483e950fc64b0315a4242

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21047" "2.21.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

❌ 2.22.x

I was unable to cherry-pick this PR to 2.22.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.22.x \
     && git checkout -b cherry-pick-21047-to-2.22.x FETCH_HEAD \
     && git cherry-pick 7970d308719df22b2cb483e950fc64b0315a4242

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21047" "2.22.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

---

When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

🤖 Beep Boop here's my run link