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

[danielinteractive]

Motivated by r-lib/urlchecker#45 (comment)

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.