ghcup install script should explicitly recommend installing command line tools on os x.

As per this thread: https://www.reddit.com/r/haskell/comments/c0djr7/how_the_fk_do_you_install_haskell/

A new haskell user was confused because they were warned about missing libs/tools, but not told that xcode and the relevant command line tools were the "standard" way to get these.