install script should check for xz before downloading

[juhp]

I tried to run curl -L --proto '=https' --tlsv1.2 https://nixos.org/nix/install in a fedora container:

[root@60d24278c208 /]# curl -L --proto '=https' --tlsv1.2 https://nixos.org/nix/install | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  2399  100  2399    0     0   3978      0 --:--:-- --:--:-- --:--:--  3985
downloading Nix 2.3.3 binary tarball for x86_64-linux from 'https://nixos.org/releases/nix/nix-2.3.3/nix-2.3.3-x86_64-linux.tar.xz' to '/tmp/nix-binary-tarball-unpack.E1mquB8Q3K'...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    86  100    86    0     0   2866      0 --:--:-- --:--:-- --:--:--  2866
100 16.4M  100 16.4M    0     0  2451k      0  0:00:06  0:00:06 --:--:-- 3626k
tar (child): xz: Cannot exec: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now
sh: failed to unpack 'https://nixos.org/releases/nix/nix-2.3.3/nix-2.3.3-x86_64-linux.tar.xz'

So it would be better if the install script complains about missing xz before downloading.

[pmiddend]

The install script seems to test for tar and then uses tar -J, but doesn't test for xz. I'll make a PR about that.