[Nix Docs] Ensure Xcode install comes before yarn cleaninstall

Summary:
The postinstall hook needs the iOS SDK to be available. Ensure that
it is installed before running 'yarn cleaninstall'

https://linear.app/comm/issue/ENG-2773

Test Plan:
From a new system, follow the instructions to install the xcode commandline tools,
Xcode, and nix. The run:

yarn cleaninstall

# ensure that the postinstall hook is working
cd native/
yarn postinstall

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6316