[ci] rename R-package GitHub Actions workflow (#3429)

* rename R-package workflow

* Revert "[ci] fix R CMD check issue 'unable to verify current time' (#3341)"

This reverts commit 0038a662.

* Revert "Revert "[ci] fix R CMD check issue 'unable to verify current time' (#3341)""

This reverts commit e2307418aa7aa7b237f0eb46f43c0535473cf462.

.github/workflows/main.yml → .github/workflows/r_package.yml

-name: GitHub Actions
+name: R GitHub Actions
 
 on:
   push: