ci(pr-labeler): run on pull_request

Run the `.github/workflows/pr-labeler.yml` action on `pull_request`
if the appropriate files have been changed, just to test the changes.

.github/workflows/pr-labeler.yml

@@ -11,6 +11,13 @@ on:
     # - https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
     types: [opened, reopened, synchronize]
 
+  pull_request:
+    # Only needed for testing: the `pull_request_target` action will also run
+    paths:
+      - .github/workflows/pr-labeler.yml
+      - .github/pr-labeler.yml
+    types: [opened, reopened, synchronize]
+
 permissions:
   contents: read