ci, gha: Ensure only a single workflow processes github.ref at a time

.github/workflows/ci.yml

@@ -7,6 +7,10 @@ on:
     tags-ignore:
       - '**'
 
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 env:
   ### compiler options
   HOST: