Usage on NixOS without Nix installed

[hacker1024]

Description

We are building NixOS systems with nix.enable = false, meaning that Nix is not installed in the final system.

The systems are designed to be completely stateless; they are network booted from a read only /nix/store mount, and use a tmpfs as the root filesystem. There is no need to use Nix profiles or generations for anything, because the system is only updated by rebooting into a new system closure (where it starts again from scratch).

At the moment, even when used as a NixOS module, Home Manager still attempts to use Nix directly in a few places in its activation script. It'd be great if there was a way to remove this dependency entirely.

Related: #6242, which was a step in this direction.