Proposal: Only run `urlchecker` on the built R package tarball

Motivated by https://github.com/r-lib/urlchecker/issues/45#issuecomment-2363632879

I think we would not want `urlchecker` to return false alarms from files that are not actually part of the R package, but just of the repository. Therefore I would propose to only run `urlchecker` on the built R package tarball, not on the overall directory as it is currently done.