error: evaluation aborted with the following error message: ‘
This version of Nixpkgs requires Nix >= 2.0, please upgrade:

- If you are running NixOS, `nixos-rebuild' can be used to upgrade your system.

- Alternatively, with Nix > 2.0 `nix upgrade-nix' can be used to imperatively
  upgrade Nix. You may use `nix-env --version' to check which version you have.

- If you installed Nix using the install script (https://nixos.org/nix/install),
  it is safe to upgrade by running it again:

      curl https://nixos.org/nix/install | sh

For more information, please see the NixOS release notes at
https://nixos.org/nixos/manual or locally at
/nix/store/d5ky0fjm70kxqw1l6vqz676g8nfz09sh-nixos-18.09/nixos/doc/manual/release-notes.

If you need further help, see https://nixos.org/nixos/support.html
’
(use ‘--show-trace’ to show detailed location information)