Skip AUR for now — attach the built Arch package to the GitHub release

Publishing to the AUR needs a maintainer AUR account and its SSH key
registered as a secret here, neither of which exists yet. Rather than
leave the workflow permanently failing at that last step, it now stops
short of AUR and instead uploads the built .pkg.tar.zst to the same
GitHub release it built from, as a plain downloadable asset (`pacman -U`
to install). The AUR-push step is still in this file's git history if
that setup happens later.

Renamed publish-aur-package.yml -> publish-arch-package.yml to match.
The render/validate steps are unchanged; new here is capturing the exact
built package filename from inside the build container (makepkg is the
only thing that actually knows it) and an upload step that follows the
same create-or-reuse-release, strip-upload_url, POST-octet-stream pattern
build-app.yml and backfill-releases.yml already use for GitHub assets,
plus a delete-existing-asset-first step so a re-dispatch for an
already-packaged version replaces rather than 422s.

Verified with a real Docker run end to end: rendered a real PKGBUILD,
built a real (synthetic) .deb through makepkg + namcap in an archlinux
container, confirmed the container exits 0, and confirmed the exact
package filename it captures (triple-c-bin-<version>-1-x86_64.pkg.tar.zst)
round-trips out via docker cp intact.

packaging/arch/PKGBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Triple-C Contributors
 #
-# This file is regenerated by .gitea/workflows/publish-aur-package.yml on every
+# This file is regenerated by .gitea/workflows/publish-arch-package.yml on every
 # publish — pkgver, the source URL and sha256sums are rewritten from the real,
 # already-uploaded release asset, never guessed. Editing pkgver/source/
 # sha256sums by hand here only matters until the next automated run overwrites