fix(release): automate PKGBUILD maintenance

Extracted `archlinux-downgrade/aur-publish-action` to handle things.

Author: pbrisbin

.github/workflows/release.yml

@@ -37,3 +37,24 @@ jobs:
         env:
           FORCE_COLOR: 1
           GITHUB_TOKEN: ${{ github.token }}
+
+    outputs:
+      published: ${{ steps.release.outputs.new_release_published }}
+      version: ${{ steps.release.outputs.new_release_version}}
+
+  pkgbuild:
+    needs: [release]
+    if: ${{ needs.release.outputs.published == 'true' }}
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: archlinux-downgrade/aur-publish-action@pb/first
+        env:
+          GIT_COMMIT_EMAIL: "${{ github.actor_id }}+${{ github.actor }}@users.noreply.github.com"
+          GIT_COMMIT_USERNAME: "${{ github.actor }}"
+          PKGNAME: downgrade
+          PKGVER: ${{ needs.release.outputs.version }}
+          PKGREL: 1 # always reset
+          PUSH: ${{ github.ref_name == 'main' && '1' || '0' }}
+          SSH_PRIVATE_KEY: ${{ secrets.SSH_PRIVATE_KEY }}