CI: head_ref -> ref, because head_ref only available on PRs

Author: sappelhoff

.github/workflows/validate_datasets.yml

@@ -12,7 +12,7 @@ on:
     - cron: "0 4 * * 1"
 
 concurrency:
-  group: ${{ github.head_ref }}
+  group: ${{ github.ref }}
   cancel-in-progress: true
 
 jobs: