devos/darwin: refine users

[astratagem]

I meant to include these changes in #438:

• Demonstrates the ability to define reusable home-manager user configurations
  via home.users and import them into any arbitrary home-manager user within a
  nixos or darwin profile.
• Rename the system/home users to primary to distinguish them from all the other
  references to 'darwin' and also to aid in demonstrating the flexibility of
  "portable" home-manager users
• Use Digga's collectHosts function to... collect hosts for deriving homeConfigurations
• Remove some extraneous comments
• Add some extraneous comments

[Pacman99]

Could we remove the nixos user and use admin/primary everywhere? It would make writing docs easier too, since we only have to reference one user to change.

[astratagem]

yeah, good point. will update.

[astratagem]

Could we remove the nixos user and use admin/primary everywhere?

I took a halfway approach, keeping the nixos user around but only using it in the bootstrap host since that's the expected user for live media.

[astratagem]

Now I'm remembering that one of the reasons I split up the users was because nix-darwin lacks a lot of the basic users.users.<name>.* options. extraGroups, for one. Builds are failing during checks for that reason. But I have some prior art from my old work MacBook that I need to resurrect anyway, so I'll move those workarounds over to these examples in the process.

https://github.com/divnix/digga/runs/7312053064?check_suite_focus=true#step:7:312