[ci] Cancel outstanding workflows when new changes are added to PRs (#513)

.github/workflows/main.yml

@@ -10,6 +10,10 @@ on:
   pull_request:
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Build ${{ matrix.artifact }}