gha: add manual build trigger to r cmd check

.github/workflows/call-r-cmd-check.yml

@@ -3,6 +3,7 @@ name: call-r-cmd-check
 # on specifies the build triggers. See more info at https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows
 on:
 # The default build trigger is to run the action on every push and pull request, for any branch
+  workflow_dispatch:
   push:
   pull_request:
   # To run the default repository branch weekly (At 12:23am on Sunday) on sunday, uncomment the following 2 lines