wrapper/build: include pynvim in python3.extraPackages by default

[Gerg-L]

this fixes using Coqtail and other plugins that require pynvim.

it's either this or set the default of vim.python3Packages to pynvim

tested with

vim = {
  luaConfigPre = ''
    vim.g.mapleader = " "
  '';
  extraPackages = [pkgs.rocq-core pkgs.coq];
  withPython3 = true;
  startPlugins = [pkgs.vimPlugins.Coqtail];
};

[github-actions]

🚀 Live preview deployed from 39345e5

View it here:

Debug Information

Triggered by: horriblename

HEAD at: python3Fix

Reruns: 2173

[horriblename]

LGTM