ci: avoid running checks for both PRs and PR branch pushes (#30)

Author: torkelrogstad

.github/workflows/check_lint_build.yaml

@@ -1,4 +1,9 @@
-on: [pull_request, push]
+
+on:
+  pull_request:
+  push:
+    branches: 
+      - master
 
 name: Check, Lint, Build