Cancel old CI jobs when a new push is done. (#2362)

Author: timcassell

.github/workflows/run-tests.yaml

@@ -5,6 +5,10 @@ on:
   pull_request:
   push:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
 
   test-windows-core: