fix: add fallback ID for scheduled test run (#2009)

Author: MarcL

.github/workflows/test.yml

@@ -8,7 +8,7 @@ on:
     - cron: '0 0 * * *'
 
 concurrency:
-  group: ${{ github.head_ref }}
+  group: ${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: