ci: add GitHub Actions workflow to validate PKGBUILD on PR

[ulises-jeremias]

Context

PRs that touch PKGBUILD/.SRCINFO should be validated in CI (namcap/makepkg --printsrcinfo dry checks where feasible in Ubuntu runners).

[ulises-jeremias]

Taking this one myself — Homebrew/AUR packaging work needs org-specific permissions (tap publish, AUR credentials, distribution secrets), so it is not a good fit for external contributors right now.

If you want to help with related docs/tests that do not require those secrets, ping me in Discord and we can carve out a safe slice.