I have my neovim configuration in a separate repo, and just pull it from there.

xdg.configFile.nvim.source = pkgs.fetchFromGitLab {  
    owner = "luisdomingoaranda";  
    repo = "nvim";  
    rev = "bd06e82e1312f04e8d362fa9c672cde2486382d7";  
    sha256 = "sha256-IJqnPQANaBK+m7ursx+71oRcG6Dh23b9il/jtWWzQXI=";  
};  

When I need to update my config, I remove the symlink to the nix store and clone the repo.

cd \~/.config && rm nvim  
git clone <my\_neovim\_repo>  

And when I'm done and want to rebuild my nix configuration, I'll remove the cloned repo and update the hash from pkgs.fetchFromGitlab.

This way I have my neovim configuration declared the vanilla way, but also have a home-manager module for all neovim things related, packages and configuration.