Avoid xcode-select --install hangups for RStudio

[zkamvar]

Based on carpentries/sandpaper-docs#33 (comment), we could implement a solution where we check if the user has macOS and are running RStudio and if so, use Sys.getenv(which -a") to check if /usr/bin/git actually works or if there is an xcrun error. If there is an error, we check if /usr/local/bin/git exists and use that version.

[zkamvar]

This is a know issue in RStudio and it seems like they don't really think it's an issue: https://community.rstudio.com/t/where-does-the-path-variable-come-from-in-sys-getenv/70744/2?u=zkamvar