/etc/nixos/configuration.nix:
{
# ...
nixpkgs.overlays = [
(import
(fetchTarball {
url = "https://github.com/TimothyKlim/nvim-flake/archive/11f4502fe38a2c79d648a0223d51b7b978589048.tar.gz";
sha256 = "1lyxjapihfvivfdxsdshrsv7rgpg3q13xqg2c20bcplvw05p9av1";
})).overlay
];
# ...
}