setup-r brew unlink r only if installed v2

[the-mad-statter]

The current version of setup-r generates an error on macos-latest due to an attempt to brew unlink r when R is not already installed via Homebrew as discussed in #864 . A reprex of this issue is available here.

This patch makes use of brew info --json r and attempts to parse the currently brew-installed R version. If a version is found, we proceed with brew unlink r and skip if no version is found.

Tests of the patch for when R is and is not installed via Homebrew are available here and here respectively.