Extracting `specialArgs.lib.nixvim` from the final `programs.nixvim`
configuration is neat, but recursive. In non-lazy scenarios, it can end
up infinitely recursive.
We could extract `specialArgs.lib.nixvim` from another nixvim
configuration, such as one of the earlier `extendModules` evaluations,
however it is better to avoid evaluating multiple configurations
unnecessarily.
Instead, revert back to getting `lib.nixvim` from the same configuration
we get `extendModules` from.