Automatically cancel outdated CI runs on PR

Author: taiki-e

.github/workflows/ci.yml

@@ -24,6 +24,10 @@ defaults:
   run:
     shell: bash
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   test:
     name: cargo test (${{ matrix.os }})