only check r cmd on push

Author: lorenzwalthert

.github/workflows/R-CMD-check.yaml

@@ -5,9 +5,6 @@
 # For help debugging build failures open an issue on the RStudio community with the 'github-actions' tag.
 # https://community.rstudio.com/new-topic?category=Package%20development&tags=github-actions
 on:
-  push:
-    branches:
-      - '*'
   pull_request:
     branches:
       - master