ci: fix merge_group checks getting canceled unexpectedly

Author: cwillisf

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: "${{ github.workflow }} @ ${{ github.head_ref || github.sha }}"
+  group: "${{ github.workflow }} @ ${{ github.event.compare || github.head_ref || github.ref }}"
   cancel-in-progress: true
 
 jobs: