CI: Limit concurrency to one per ref_name (#1275)

This should limit concurrency to only a single workflow run per branch (including
main) to run at the same time.

If we're landing multiple commits via merge queue, it should force the
newest one to be tested only.

Author: LeonMatthesKDAB

.github/workflows/github-cxx-qt-tests.yml

@@ -26,7 +26,7 @@ on:
 
 # Cancel any previous runs for the same pull request
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
+  group: ${{ github.workflow }}-${{ github.ref_name || github.run_id }}
   cancel-in-progress: true
 jobs:
   clang_format: