Issue #33: New push cancels old runs.

Author: MattClarkson

.github/workflows/build.yml

@@ -7,6 +7,10 @@ on:
   pull_request:
     branches: [master, main]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   # =========================================================================
   # C++ tests (quick feedback, no wheel building)