Revert "ci: use head_ref for concurrency group"

This reverts commit e6e6d0c.

(cherry picked from commit c9ce423)

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
     types: [opened, synchronize, reopened]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 permissions: {}