How to package language servers with nvim override?

I think it may be a little easier if you use home-manager, which has programs.neovim.extraPackages.

Although I believe you could simply also add these to your user packages if you’re willing to make them available everywhere in your user’s environment.

    users.users.your-username-goes-here.packages = with pkgs; [
      # Language servers
      rnix-lsp
      haskell-language-server
      sumneko-lua-language-server
      elmPackages.elm-language-server
    ];