neovim-flake/docs/manual/configuring/custom-package.md

Custom Neovim Package

As of v0.5, you may now specify the Neovim package that will be wrapped with your configuration. This is done with the vim.package option.

{inputs, pkgs, ...}: {
  # using the neovim-nightly overlay
  vim.package = inputs.neovim-overlay.packages.${pkgs.system}.neovim;
}

The neovim-nightly-overlay always exposes an unwrapped package. If using a different source, you are highly recommended to get an "unwrapped" version of the neovim package, similar to neovim-unwrapped in nixpkgs.

{ pkgs, ...}: {
  # using the neovim-nightly overlay
  vim.package = pkgs.neovim-unwrapped;
}