nixos-install: Make compatible with Nix 2.0

The use of Nix 2.0 significantly simplifies the installer, since we
can just pass a different store URI (--store /mnt) - it's no longer
needed to set up a chroot environment for the build, and to bootstrap
Nix into the chroot.

Also, commands that need to run in the installation (namely boot
loader installation and setting a root password) are now executed
using nixos-enter.

This also removes the need for nixos-prepare-root since any required
initialisation is done by Nix or by the activation script.