./github/workflows/add-label-when-promoted.yaml: Run auto-backport only on default branch

In https://github.com/scylladb/scylladb/pull/21496#event-15221789614
```
scylladbbot force-pushed the backport/21459/to-6.1 branch from 414691c to 59a4ccd Compare 2 days ago
```

Backport automation triggered by `push` but also should either start from `master` branch (or `enterprise` branch from Enterprise), we need to verify it by checking also the default branch.

Fixes: https://github.com/scylladb/scylladb/issues/21514

Closes scylladb/scylladb#21515

.github/workflows/add-label-when-promoted.yaml

@@ -49,7 +49,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/label_promoted_commits.py  --commits ${{ github.event.before }}..${{ github.sha }} --repository ${{ github.repository }} --ref ${{ github.ref }}
       - name: Run auto-backport.py when promotion completed
-        if: github.event_name == 'push'
+        if: github.event_name == 'push' && github.ref == 'refs/heads/${{ env.DEFAULT_BRANCH }}'
         env:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/auto-backport.py --repo ${{ github.repository }} --base-branch ${{ github.ref }} --commits ${{ github.event.before }}..${{ github.sha }}