nix-env: Refuse to operate on a new-style profile
nix-env: Refuse to operate on a new-style profile

This prevents users from accidentally nuking their profile via
nix-env.

rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix