fix(packaging): port PKGBUILD contract files to the canonical line (Refs #316, #342)

[AlexMikhalev]

Summary

Two bugs in the Arch/AUR PKGBUILD contract added by #343, found when the workflow first ran on GitHub:

1. PR ci(packaging): mirror pkgbuild-contract workflow to the canonical line (Refs #316, #342) #36 mirrored only .github/workflows/pkgbuild-contract.yml to the canonical GitHub line, not pkgbuilds/terraphim-clients-bin/** or tests/test_pkgbuild_release_contract.py. The workflow therefore ran against files that do not exist on GitHub — the contract job failed with ModuleNotFoundError: No module named tests.test_pkgbuild_release_contract and the Arch job with ./pkgbuilds/terraphim-clients-bin/verify.sh: No such file or directory.
2. The contract step used python3 -m unittest tests.test_pkgbuild_release_contract, which requires tests/ to be an importable package (there is no tests/__init__.py). Switched to python3 -m unittest discover -s tests -p "test_pkgbuild_release_contract.py" -v, matching the canonical ci.yml style.

This ports the full #343 content to the canonical line and fixes the invocation.

Evidence

$ python3 -m unittest discover -s tests -p "test_pkgbuild_release_contract.py" -v
Ran 7 tests ... OK (skipped=1)

Refs terraphim-clients#316, #342.