workflows/labeler: do not set labels on stable backport PRs

(cherry picked from commit 5dfc88c)

Author: YHNdnzj

.github/workflows/labeler.yml

@@ -35,7 +35,7 @@ jobs:
 
     - name: Label PR based on policy in labeler.yml
       uses: actions/labeler@8558fd74291d67161a8a78ce36a881fa63b766a9
-      if: startsWith(github.event_name, 'pull_request') && github.event.action != 'closed'
+      if: startsWith(github.event_name, 'pull_request') && github.base_ref == 'main' && github.event.action != 'closed'
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"
         configuration-path: .github/labeler.yml