[Nix Docs] Install Xcode before cli tooling

Summary:
Xcode will also install the iOS SDK, which is needed for the
postinstall hooks to complete successfully. Installing Xcode before
the cli tools ensures that the cli tools can inherit the installed SDK.

https://linear.app/comm/issue/ENG-3037

Test Plan: N/A

Reviewers: ashoat, varun, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6766