Can't find tests if using a `path_prefix`

[tykeal]

This was working when the source was in /src/ and the tests were in /tests/ but for various reasons they needed to be moved down one directory and. When running the action without supplying the path_prefix it outright failed not being able to determine anything related to the python versions. Once adding path_prefix now it fails to find the tests.

Failing PR https://github.com/tykeal/homeassistant-rentalsync-bridge/actions/runs/21689430197/job/62545104983?pr=30

[ModeSevenIndustrialSolutions]

Testing here: lfit/dependamerge#174

[ModeSevenIndustrialSolutions]

@tykeal Please review: #139

A subsequent workflow update will then follow, which I'll apply to the dependamerge repository. You can then copy that workflow into tykeal/homeassistant-rentalsync-bridge, rebase, and try again.

The workflow update here is in draft: lfit/dependamerge#174

Will need updating once we've tagged and released an update to python-build-action.