Rename `nix show-config` to `nix config show`

Part of #7672

My main motivation is to be able to use `nix.checkConfig`[1]. This
doesn't work with Lix currently since the module uses `nix show-config`
if the Nix version is <2.20pre and `nix config show` otherwise. I think
this is the only instance where nixpkgs checks for which Nix commands
exist that affects us now, so I figured we could just perform the rename
here as well[2] and still provide the current version number[3].

I don't have a strong opinion on whether to deprecate `nix show-config`,
the warning is added there automatically.

(cherry picked from commit f300e11b056dea414d7d77bbc6e5a7dc5d9ddd41)

[1] https://nixos.org/manual/nixos/stable/options.html#opt-nix.checkConfig
[2] I should add that I don't use the "official" ways of installing Lix
    because using the flake directly and callPackaging it seemed to fit
    better into my workflow: I already have a little mess to make
    sure Hydra from the flake uses the correct pkgs.nix and I didn't
    want to complicate it further while keeping a single package-set I
    can build in CI. Don't get me wrong, I think such a module for a
    quick-start is very important, just giving context on why I bother
    in the first place :)
[3] When we go public, I think it's worth considering to add support in
    nixpkgs itself for Lix.

Change-Id: I47b4239b05cbeda3c370d2fa56ea768b768768ac

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