ci: concurrency

Author: Borda

.github/workflows/ci_testing.yml

@@ -4,6 +4,10 @@ name: CI testing
 on:  # Trigger the workflow on push or pull request
   pull_request: {}
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
 jobs:
   nbval:
     runs-on: ${{ matrix.os }}