Add workflow dispatch to R-CMD-check.yaml

Author: cderv

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

@@ -5,6 +5,7 @@
 # check-standard.yaml is likely a better choice.
 # usethis::use_github_action("check-standard") will install it.
 on:
+  workflow_dispatch:
   push:
     branches: [main, master]
   pull_request: