Cancel previous CI runs when PR is repushed (#315)

see: https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-concurrency-to-cancel-any-in-progress-job-or-run

Author: asherf

.github/workflows/ci.yml

@@ -6,6 +6,9 @@ on:
   pull_request:
     branches:
       - master
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
 jobs:
   test:
     timeout-minutes: 30