Run CI build only for master branch

This change updates the CI build workflow to only trigger on pull
requests to the master branch. The release branches almost never cause
build errors, so we can skip that check.

.github/workflows/build-trusted.yml

@@ -1,11 +1,13 @@
 name: Kernel Build (Trusted)
 
-on: pull_request
+on:
+  pull_request:
+    branches: [master]
 
 jobs:
   build:
     name: Build
-    runs-on: [self-hosted]
+    runs-on: self-hosted
     if: github.event.pull_request.head.repo.full_name == github.repository && !contains(github.event.head_commit.message, 'skip ci')
     steps:
     - name: Clean Workspace