The act_runner service account doesn't have python in PATH. Delegating
to actions/setup-python@v5 which downloads/caches Python 3.11 reliably.
Also bump timeout-minutes 15 -> 20 to absorb first-run pip install.
Gitea Actions only scans .gitea/workflows/ and .github/workflows/. The
file under gitea/workflows/ (no leading dot) was never picked up. Other
files in gitea/workflows/ remain as templates copied into consumer repos.