Optionally install GHC using musl alongside

If ghcup is to be a management tool for GHCs (à la rustup), this would be a wonderful feature.

We already have musl tarballs for alpine https://gitlab.haskell.org/haskell/ghcup/blob/master/.download-urls#L92-93

But they are not exposed for non-alpine versions. I'm currently not sure what the best way would be, because we would have to circumvent the OS detection.

mentioned in commit 02e35666

mentioned in merge request !131 (merged)

closed via merge request !131 (merged)