Revert "make-disk-image: replace nixos-install by nix-env calls"
Revert "make-disk-image: replace nixos-install by nix-env calls"

This reverts commit e9bf955fd6d5ffda218878ba561c93c6a28dfe8e. We use
nixos-install to ensure that make-disk-image produces the same result
as a regular installation (9802da517fe4da41d68516029b01c5c1f175b3e4)
and to reduce code duplication. If there is something broken in
nixos-install, it should be fixed there.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs