only apply style on master

Author: Rafnuss

.github/workflows/style.yaml

@@ -2,6 +2,8 @@
 # Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
 on:
   push:
+    branches:
+      - master
     paths: ["**.[rR]", "**.[qrR]md", "**.[rR]markdown", "**.[rR]nw", "**.[rR]profile"]
 
 name: style.yaml