ci(gh-actions): limit ci actions to master branch

This causes issues when a release is published, because the build scripts create a new branch
because of the new tag that gets automatically pushed by GitHub

.github/workflows/label-sync.yml

@@ -1,6 +1,8 @@
 name: Sync labels
 on:
   push:
+    branches:
+      - master
     paths:
       - '.github/labels.yml'
   workflow_dispatch: