CI: set concurrency limits to avoid extra builds

.github/workflows/cmake.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [ "main" ]
 
+concurrency:
+  group: cmake-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: ${{ matrix.os }}