Restore concurrency updates

Author: LilithHafner

.github/workflows/CI.yml

@@ -6,6 +6,13 @@ on:
       - main
     tags: '*'
 
+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 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/') }}
+
 # needed to allow julia-actions/cache to delete old caches that it has created
 permissions:
   actions: write