Update build.yml

Author: wopian

.github/workflows/build.yml

@@ -1,6 +1,14 @@
 name: Build
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    # Branches from forks have the form 'user:branch-name' so we only run
+    # this job on pull_request events for branches that look like fork
+    # branches. Without this we would end up running this job twice for non
+    # forked PRs, once for the push and then once for opening the PR.
+    branches:
+    - '**:**'
 
 jobs:
   build: