installer: also test for xz to unpack

[pmiddend]

As mentioned in #3430, we are checking for tar, and then use -J to unpack the xz file. But this needs xz, too!

[LnL7]

On some platforms, like darwin, tar supports lzma commpression without having xz available as a command. This change makes the command mandatory which means people would have to install another package manager to make that available first.

$ xz --help
The program 'xz' is currently not installed. You can install it
by typing:
  nix-env -iA nixpkgs.lzma.bin

$ /usr/bin/tar -v -xJf nix-2.3.5-x86_64-darwin.tar.xz -C /tmp
x nix-2.3.5-x86_64-darwin/install
x nix-2.3.5-x86_64-darwin/create-darwin-volume.sh
x nix-2.3.5-x86_64-darwin/install-darwin-multi-user.sh
x nix-2.3.5-x86_64-darwin/install-systemd-multi-user.sh
x nix-2.3.5-x86_64-darwin/install-multi-user
...

     -J, --xz
             (c mode only) Compress the resulting archive with xz(1).  In extract or list modes, this option is
             ignored.  Note that, unlike other tar implementations, this implementation recognizes XZ compression auto-
             matically when reading archives.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/trouble-installing-nix-on-ubuntu-20-04-3-lts/15085/2