ci: use pull_request_target

.github/workflows/check-commit-signing.yaml

@@ -1,8 +1,7 @@
 ---
 name: Check if commits are signed
 on:
-  # pull_request_target:
-  pull_request:
+  pull_request_target:
     branches: [main]
 concurrency:
   group: ${{ github.workflow }}--${{ github.head_ref }} # github.ref is unavailable in case of pull_request_target