Switch to github.head_ref for concurrency key

It looks like `github.ref` points to the target branch when triggered by
`pull_request_target`, while `github.head_ref` is always the source
branch.

.github/workflows/check-labels.yaml

@@ -8,7 +8,7 @@ on:
     types: [opened, synchronize, labeled, unlabeled]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.head_ref }}
   cancel-in-progress: true
 
 jobs: