Fix workflow permission issue when running in merge branch (sonic-net…

…#7417)

Fix the labeler workflow permission issue when merging from fork repo.
It impacts the labeler workflow to support auto-merge for package versions upgrade on 202012 branch. The current workaround is to add the label "automerge" on the PR sent by mssonicbld, then the automerge workflow will merge the PR.

.github/workflows/label.yml

@@ -7,7 +7,7 @@
 
 name: Labeler
 on:
-  pull_request:
+  pull_request_target:
     types:
     - opened
     - reopened