[Nix] Update Nix support for Mina respecting o1js submodule inversion

[shimkiv]

The o1js is no longer submodule of Mina repo (https://github.com/o1-labs/o1js/issues/1096) yet Nix configuration contains leftovers that will be goo to clean up. At the same time we do still need to be able to build o1js_stubs within the Mina repo which depends on the Wasm bindings.

[shimkiv]

Related internal discussion thread.