[ci] Cancel outstanding workflows when new changes are added to PRs (#571)

.github/workflows/main.yml

 name: CI
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   buildlibc:
     name: Build libc