fix(ci): only trigger build for push events on master branch

Avoids running all jobs twice for pushes to pull requests.

Author: sgarner

.github/workflows/build.yml

@@ -2,7 +2,7 @@ name: Build
 
 on:
   push:
-    branches: [ '**' ]
+    branches: [ 'master' ]
   pull_request:
     branches: [ '**' ]