# ./configuration.nix
environment.systemPackages =
    let nvimPackages = (import ./customisation/nvimPackages.nix pkgs);
    in nvimPackages ++ (with pkgs;
    [ wget
...
# ./customisation/nvimPackages.nix
let nvim = pkgs.neovim.override {
             vimAlias = true;
             configure = {
               customRC = let vimrc = builtins.readFile ./init.vim;
                          in ''${vimrc}'';
             };
           };
in with pkgs;
   [ nvim