Update Github action to use pull_request_target from

https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target.

The existing behavior only triggered the action for PRs from the repo and not
PRs from forks, which made its usefulness minimal.

PiperOrigin-RevId: 325286854

Author: alanchiao

.github/workflows/labeler.yml

@@ -14,7 +14,7 @@
 
 name: "Pull Request Labeler"
 on:
-- pull_request
+- pull_request_target
 
 jobs:
   triage: