Use pull_request_target so action works for contributor PRs

Author: marcphilipp

.github/workflows/label-pull-request.yml

@@ -1,6 +1,6 @@
 name: Copy labels from linked issues to PR
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, reopened, edited, synchronize]
 permissions: {}
 jobs: