./github/workflows/add-label-when-promoted.yaml: Run auto-backport on…

…ly on default branch

In #21496 (comment)
```
scylladbbot force-pushed the backport/21459/to-6.1 branch from 414691c to 59a4ccd Compare 2 days ago
```
Backport automation is triggered by push but should also start from the master branch (or enterprise branches from Enterprise). We need to verify it by checking the default branch.

(cherry picked from commit 26ef84f)

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

@@ -40,6 +40,7 @@ jobs:
         run: |
           git config --global user.name "GitHub Action"
           git config --global user.email "[email protected]"
+          git config --global merge.conflictstyle diff3
       - name: Install dependencies
         run: sudo apt-get install -y python3-github python3-git
       - name: Run python script
@@ -48,7 +49,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.AUTO_BACKPORT_TOKEN }}
         run: python .github/scripts/search_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 }}