nixos/common: forward nix.enable from the OS configuration

[emilazy]

Description

This prevents problematic references to nix.package when Nix management is disabled in the host OS.

Drafted until the corresponding nix.enable functionality is merged into nix-darwin, since this is probably more useful there than on NixOS.

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Note: This work was funded by Determinate Systems. The design is my own.

Checklist

• Change is backwards compatible. (‐ish; you could theoretically be relying on the host OS’s nix.package when nix.enable is turned off, but that sounds unwise…)

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example. (it doesn’t seem like there are any existing tests for nix.enable, so not sure where to add this)

• 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.

Maintainer CC

@rycee