Improve workflow

.github/workflows/build.yml

@@ -1,11 +1,12 @@
 name: Node.js Build
 
-on:
-  - push
-  - pull_request
+on: [ push, pull_request ]
 
 jobs:
-  cache-and-install:
+  build:
+    # Only run on PRs if the source branch is on someone else's repo
+    if: "${{ github.event_name != 'pull_request' || github.repository != github.event.pull_request.head.repo.full_name }}"
+
     runs-on: ubuntu-latest
 
     steps: