nh: validate nix features & version

[NotAShelf]

Isolated from #258. We now check for a minimum Nix version (should be equal to nixVersions.latest from the stable branch) and required Nix features (flakes and nix-command) as a part of our startup checks, which previously only checked for NH_FLAKE or/and FLAKE being set. This also moves the checks out of main and to a dedicated module for future maintainability reasons.

Closes #237

[NotAShelf]

@drupol (pinging as the other nh maintainer)

This new change causes fixupPhase to fail during nix build because the testing sendbox does not have the required features check. Do you think we can set the required variables (or disable the check via NH_NO_CHECKS) only during build? env.NH_NO_CHECKS would do for suppressing the warning, but I believe that'll disable the functionality entirely even after build.

[drupol]

Hello,

I don't have a strong opinion on this yet, do what you think is better, and ping me when it's done, I'll have a look.

[NotAShelf]

Apparently the checks were failing because we ran installations in preFixup. Moving those to postInstall appears to have fixed the build fail.

[andersk]

This does not work with Determinate Nix, for which nix-command and flakes are not considered experimental (even if explicitly listed in extra-experimental-features).

$ nix --version
nix (Determinate Nix 3.6.1) 2.29.0
$ cat ~/.config/nix/nix.conf 
# WARNING: this file is generated from the nix.settings option in
# your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
# Do not edit it!
extra-experimental-features = nix-command flakes
extra-substituters = https://nix-community.cachix.org
extra-trusted-public-keys = nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=
$ nix config show experimental-features

$ nh home switch .
! Missing required Nix features: nix-command, flakes (nh/src/checks.rs:101)
Error: 
   0: Missing required experimental features. Please enable: nix-command, flakes

Location:
   src/checks.rs:105

[NotAShelf]

We only support mainsteam Nix, and Lix to a degree.

[viperML]

It shouldn't be too hard to support Dix for this. In any case we won't try too hard to support Dix if it start to diverge too mucho from Nix for other things...