Switched to pull_request_target event trigger in the Labels Github action.

The `pull_request` trigger runs in the context of the PR, and has
limited access to information stored in the base repo django/django.
As access is needed to the label "no ticket" in django/django, use the
`pull_request_target` trigger.

.github/workflows/labels.yml

@@ -1,7 +1,7 @@
 name: Labels
 
 on:
-  pull_request:
+  pull_request_target:
     types: [ edited, opened, reopened, ready_for_review ]
 
 concurrency: