[nix] Replaces brew install of watchman with nix

Summary:
Because we updated nix packages, we can use nix-based watchman instead of the brew install

Context: https://linear.app/comm/issue/ENG-2784/add-updated-nix-version-of-watchman#comment-79fd27f8

Test Plan:

 ➜ whereis watchman
watchman: /nix/store/pmvk9zd5z98b5vvc83w4dqjq7d4w8ri1-watchman-2023.08.14.00/bin/watchman

To verify the new Nix watchman is working. I uninstalled brew-based watchman and ran nix develop. I then ran the iOS simulator and made changes to text on the login screen, verifying that changes were immediate

Reviewers: ashoat, bartek, varun, O1 Nix

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12777