Re-enable CI builds for release branches

This reverts commit 01cbdebf0a.
Although almost all release builds succeed, there have been cases where
new stable releases actually cause the build to fail or other cases
where we need to test changes on the release branch.

.github/workflows/build-trusted.yml

@@ -1,13 +1,11 @@
 name: Kernel Build (Trusted)
 
-on:
+on: pull_request
-  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