chore(ci): improve error handling and push only ref

.gitea/workflows/sync.yaml

@@ -32,7 +32,7 @@ jobs:
         EOF
     - name: Push
       run: |
-        git push ssh://aur@aur.archlinux.org/mint-backgrounds.git ${{ github.ref }}
+        git push ssh://aur@aur.archlinux.org/mint-backgrounds.git 'HEAD:refs/heads/master'
     - name: Remove SSH client configuration
       run: |
         rm -rf ~/.ssh