Add fallback value to concurrency.group (#2391)

For runs on `master`

Author: martijnbastiaan

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ${{ github.head_ref }}
+  group: ${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: