Restrict push trigger to master to avoid duplicate CI runs

Previously every branch push triggered both the push and pull_request
workflows when a PR was open. Limiting push to master means branch
pushes only trigger via pull_request.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/build.yml

@@ -2,6 +2,8 @@ name: Build
 
 on:
     push:
+        branches:
+            - master
     pull_request:
     release:
         types: [created]