Move builtins as nix plugin in into this repo.

[hsjobeki]

I want to move the c++ stuff into a plugin.
I want to ship a fixed nix version that is strapped together with the plugin to ensure stability, since we need to access low level functions that are not stable Api of nix. And have changed in newer versions.

This is a temporary solution that improves build time and potentially unlocks noogle as a cli tool until the tvix-evaluator is ready enough.