ci: add needs:backport label on backport PRs with conflict

This makes it easy to keep track of which backport PRs have failed and
need manual intervention to fix.

(cherry picked from commit 4c23b83456)

.github/workflows/backport.yml

@@ -27,6 +27,18 @@ jobs:
           label_pattern: "^ci:backport ([^ ]+)$"
           github_token: ${{ steps.app-token.outputs.token }}
 
+      - name: Create failed backport label
+        if: ${{ steps.backport.outputs.was_successful == 'false' }}
+        uses: actions/github-script@v7
+        with:
+          script: |
+            github.rest.issues.addLabels({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              labels: ['needs:backport']
+            })
+
       - name: Enable automerge
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}