CI: update concurrency check

Author: fingolfin

.github/workflows/CI.yml

@@ -9,8 +9,8 @@ on:
 
 concurrency:
   # group by workflow and ref; the last slightly strange component ensures that for pull
-  # requests, we limit to 1 concurrent job, but for the master branch we don't
-  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/master' || github.run_number }}
+  # requests, we limit to 1 concurrent job, but for the default repository branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref_name != github.event.repository.default_branch || github.run_number }}
   # Cancel intermediate builds, but only if it is a pull request build.
   cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}