Older versions of tar don't understand the `-xf` flags while newer
versions understand the "bundled-flags" (as documend in bsdtar's
manpage) perfectly. So `tar xf` should maximize compatibility.

At least as of Mac OS 10.13.6 the included bsdtar has no flag for xz
decompression while `z` means gzip decompression which is obviously
problematic for xz compressed archives.

I believe that any tar version on Mac OS that can decompress xz archives
also automatically detects compression formats. So `tar xf` should be
fine.

Fixes #119

This is redundant to build-essential.

If libtinfo5 isn't installed, then installation fails with a message like this:
/home/ubuntu/.ghcup/ghc/8.6.5/lib/ghc-8.6.5/bin/ghc-pkg: error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory

If xz-utils isn't installed, then installation fails with a message like this:
Following commands are required, but missing, please install:  xz

Fixes #113

Fixes #112

xz is not needed on darwin and checking for it generates a lot
of questions for new users that we could easily avoid.

It's only needed for building GHC.