fix: Update workflow to use REGISTRY_USERNAME and REGISTRY_PASSWORD secrets

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -21,8 +21,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: gitea.likegears.com
-          username: ${{ gitea.actor }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
-          password: ${{ secrets.REGISTRY_TOKEN }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Extract version from tag
         id: version