Remove github.event.pull_request.head.label and github.head_ref as th…

…ey aren't defined on push events

.github/workflows/build_branch.yaml

@@ -23,7 +23,7 @@ on:
 
 # This allows a subsequently queued workflow run to interrupt previous runs
 concurrency:
-  group: '${{ github.workflow }} @ ${{ github.event.pull_request.head.label || github.head_ref || github.ref }}'
+  group: '${{ github.workflow }} @ ${{ github.ref }}'
   cancel-in-progress: true
 
 permissions: