Change workflow trigger from pull_request to pull_request_target

This allows the workflow to access secret variables which are not accessible
in pull_request events.

[ci skip]

Author: dahlia

.github/workflows/ai-labeler.yaml

@@ -2,7 +2,7 @@ name: ai-labeler
 on:
   issues:
     types: [opened, edited, reopened, labeled]
-  pull_request:
+  pull_request_target:
     types: [opened, edited, reopened, labeled]
 
 jobs: