CI: Switch the labeler event to pull_request_target

Otherwise, the action won't work on PRs from forked repositories
(#10326).

.github/workflows/label.yml

@@ -10,7 +10,7 @@
 # https://github.com/actions/labeler
 
 name: Labeler
-on: [pull_request]
+on: [pull_request_target]
 
 jobs:
   label: