Use fallback value

.github/workflows/build.yaml

@@ -2,8 +2,8 @@ name: build
 
 on: [push, pull_request]
 
-concurrency: 
-  group: ${{ github.head_ref }}
+concurrency:
+  group: ${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs:

.github/workflows/test.yaml

@@ -2,8 +2,8 @@ name: test
 
 on: [push, pull_request]
 
-concurrency: 
-  group: ${{ github.head_ref }}
+concurrency:
+  group: ${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: