[Nix] Move to rustup supplied tooling

Summary:
For compatibility with iOS target compilation, we need to avoid nix
provided rust tooling which only supports targeting the build platform.

Test Plan:

nix develop

which rustc # points to rustup package in nix store

yarn cleaninstall # may not be necessary

cd native
yarn react-native run-ios --simulator='iPhone 14 Pro' --configuration=Release

Depends on D5479

Reviewers: atul, varun, O1 Nix

Reviewed By: varun, O1 Nix

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5509