ci: stop running build twice on PRs from in-repo branches

Scope the `push` trigger to `master` only. Previously both `push` and
`pull_request` fired for every push to a feature branch with an open PR,
duplicating each Quartus build.

Author: asturur

.github/workflows/ci_build.yml

@@ -3,6 +3,7 @@ name: CI Build
 on:
   pull_request:
   push:
+    branches: [master]
   workflow_dispatch:
 
 permissions: