We already have the latest version available quite reliably.

(Apple rebranded Mac OS X to OS X in 2012, and then to macOS in 2016.)

Fixes #73

Added downloads available from https://downloads.haskell.org/~ghc/8.6.4/
as at the time of commit.

Have not made this recommended yet until common tools and libs are all
confirmed to work fine.

This seems to have caused occasional weird
side-effects on the following line:

  edo curl https://raw.githubusercontent.com/haskell/ghcup/master/ghcup > "${HOME}"/.ghcup/bin/ghcup
  ...
  edo ghcup install

It would result in:

  ~/.ghcup/bin/ghcup: line 1: $'\E[0': command not found
  ~/.ghcup/bin/ghcup: line 1: 35mcurl: command not found

which seems to be because of printf and the piping command.
It's not entirely clear to me what happens here, but it
seems part of the printf output to stdout where mixed
with the next command.

It seems to work well with current ecosystem and stackage
has just introduced 8.6.3 as part of lts-13.1:
  https://www.stackage.org/lts-13.1

In most cases, when we run a subshell with our own function
and assign the result to a variable, we also need to check that
the variable is non-empty, since 'die' and 'edo' don't propagate
to the parent shell.

In some cases, non-emptiness is handled in other ways or is not
fatal.

If a GHC is already installed, we shouldn't treat it as an
error.