[ci skip] Don't run build twice

.github/workflows/build.yml

@@ -4,6 +4,7 @@ on: [pull_request, push]
 
 jobs:
   build:
+    if: ${{ github.event_name != 'pull_request' || github.repository != github.event.pull_request.head.repo.full_name }}
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Repository