fix(ci): use gpg-agent loopback instead of custom sign command

The custom %__gpg_sign_cmd macro with %{__plaintext_filename} is not
supported on modern rpm. Instead, configure gpg-agent for loopback
pinentry and let rpm use its default sign command.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-release.yml

@@ -137,6 +137,8 @@ jobs:
         run: |
           echo "${{ secrets.RPM_SIGNING_KEY }}" | gpg --batch --import
           sed "s/@GPG_NAME@/${{ secrets.RPM_SIGNING_KEY_ID }}/" rpm/rpmmacros > ~/.rpmmacros
+          echo "allow-loopback-pinentry" >> ~/.gnupg/gpg-agent.conf
+          gpgconf --kill gpg-agent
 
       - name: Sign and publish
         run: ./script/publish-repo.sh rpms/