Prevent backport workflow from running on umerged PRs

Signed-off-by: Miki <amoo_miki@yahoo.com>

.github/workflows/backport.yml

@@ -12,6 +12,17 @@ jobs:
       contents: write
       pull-requests: write
     name: Backport
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
     steps:
       - name: GitHub App token
         id: github_app_token