Enable labeler action again

This time using `pull_request_target`, so it will work properly with forks. See
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target

This reverts commit d5d6093e75.

.github/workflows/labeler.yml

@@ -0,0 +1,11 @@
+name: "Pull Request Labeler"
+on:
+- pull_request_target
+
+jobs:
+  triage:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@v2
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"