ci: update PR event trigger to pull_request_target

Ref:
- https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows
- https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

.github/workflows/tests.yml

@@ -6,7 +6,7 @@ on:
   push:
     branches:
       - main
-  pull_request:
+  pull_request_target:
     branches:
       - main
 jobs: