• use ./install.sh to install globally via nix profile
• use nix build '.#nvim' to build locally (you will generate a ./result symlink
• to incoporate into configuration.nix:
  1. Add flake source to your flake.nix, supposing:

     inputs.nixvim.url = "./modules/nixvim-config";

  2. Add the flake into your configuration.nix (ensure system and inputs are passed in via extraSpecialArgs in flake file):

     environment.systemPackages = [ inputs.nixvim.packages.${system}.default ];

I suggest testing locally with just nix build and result/bin/nvim, and once happy, rebuild your nixos.