ci: add concurrency

Author: ikreymer

.github/workflows/ci.yaml

@@ -2,6 +2,10 @@ name: CI
 
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   unit-tests:
     timeout-minutes: 10