Don't cancel concurrent CI builds running for merged commits on master

This would happen for the commit that was merged first:
> Canceling since a higher priority waiting request for 'ci-' exists

More info:
https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value

Author: ruohola

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ci-${{ github.head_ref }}
+  group: ci-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: