make CI work with fork PRs

.github/workflows/build.yml

@@ -4,9 +4,11 @@ on:
   push:
     branches: [ main ]
   pull_request:
+  pull_request_target:
 
 jobs:
   build:
+    if: ${{ github.event_name == 'push' || github.event_name == 'pull_request' && !github.event.pull_request.head.repo.fork || github.event_name == 'pull_request_target' && github.event.pull_request.head.repo.fork}} 
     runs-on: ubuntu-latest
     steps:
       - name: Checkout