system/checks: allow disabling the buildUsers check

[cole-h]

This makes it possible for people who are using the auto-allocate-uids experimental feature (available since 2.12.0) to install nix-darwin! Otherwise, this check will fail because, predictably, there are no build users as they're allocated at build-time.

[cole-h]

Now that I think about it, it might be better to just allow disabling this check altogether rather than doing heuristics that may not hold.

[roman]

@domenkozar can we prioritize merging this, please? ☝️

On newer installs via DeterminateSystems/nix-installer nix-darwin fails miserably

[bestlem]

I think the default has to be don't do the check.

The reason is how do you turn the check off - that is not in the documentation.

I ended up commenting out the check line. as even with this patch it still fails.

[cole-h]

You turn the check off by setting system.checks.verifyBuildUsers = false; in your nix-darwin config somewhere.

[bestlem]

The install I was doing was install nix and then nix-darwin and only then look at config.

[cole-h]

You'll want to follow the instructions for installing nix-darwin from here: https://github.com/LnL7/nix-darwin#flakes-experimental

Specifically, you create your nix-darwin config, then you build it on your darwin machine, and then you use the darwin-rebuild script in that built darwin configuration to complete the switch.