(MAINT) Sync checklist GHA

The checklist GHA in this repository used `pull_request` instead
of `pull_request_target` for the trigger. This change updates it
to use the correct trigger used by the core docs repository.

.github/workflows/checklist.yml

@@ -1,6 +1,6 @@
 name: Checklist
 on:
-  pull_request:
+  pull_request_target:
     branches:
       - main
     types: