nix: add structural settings

[polykernel]

Nix permits user level configurations through ~/.config/nix/nix.conf that allow
customization of system-wide settings and behavior. This is beneficial in chroot
environments and for per-user configurations. System level Nix configurations in the
form of /etc/nix/nix.conf can be specified declaratively via the NixOS nix module but as
of currently no counter part exists in home-manager.

This PR is a port of the RFC42 implementation for the NixOS nix module[1]
to home-manager. Non-applicable options have been excluded and the config generation
backends have been tweaked to the backends offered by home-manager. A notable change
from the NixOS module is a mandatory option to specify the Nix binary corresponding
to the version "nix.conf" should be generated against. This is necessary because
the validation phase is dependent on the nix show-config subcommand on the host platform.
While it is possible to avoid validation entirely, the lack of type checking was deemed too significant.
In NixOs, the version information can be retrieved from the package option itself which
declares the Nix binary system-wide. However in home-manager, there is no pure way to
detect the system Nix version and what state version the "nix.conf" should be generated
against. Thus an option is used to overcome this limitation by forcing the user to
specify the Nix package. Note this interaction can still be automated by forwarding
the system-wide Nix package to the home-manager module if needed.

Three unit tests were added to test the module behavior for the empty settings, the example
settings and the example registry configurations respectively.

[1] - NixOS/nixpkgs#139075

Description

Efforts towards #2324

Todo

• Write unit tests

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

  • Added myself and the module files to .github/CODEOWNERS.

[berbiche]

Thanks!

The only required change is changing home-manager for Home Manager, the other comments may be ignored.

[berbiche]

Suggested change

	# your home-manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
	# your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.

[polykernel]

I wasn't sure on the correct naming convention. Thanks!

[polykernel]

Sorry for the delay, some life events got in the way. I have applied the required changes as requested.

[berbiche]

Thanks!

[ncfavier]

This does not trigger when only extraOptions is set, causing #3644.

Also, is there a reason not to set pkgs.nix as the default package?

[polykernel]

The reason pkgs.nix is not set as the default is because the config generation checkPhase depends on the system nix version. The concern was that if the version of default nix package did not match the user's installed nix version, then the show-config nix command could potentially validate against an incompatible config schema. This is an issue when new releases introduce option changes that break backwards compatibility(e.g options being renamed/removed).

We can't assumed that home-manager has access to system state(i.e as a NixOS module) to determine the system nix version so the compromise was made to force the user to determine the version/schema to be used.

[ncfavier]

Fair enough, but we could at least try osConfig.nix.package

EDIT: I guess if that is set then the user probably doesn't want to manage nix per-user...

[ncfavier]

#3647