Cancel in-progress CI runs when new code is pushed

Author: warting

.github/workflows/build-test.yml

@@ -8,6 +8,10 @@ on:
     paths-ignore:
       - '**.md'
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: ubuntu-latest