[Nix] Install watchman through homebrew

Summary:
'watchman' on nixpkgs is very old; until it has
been updated, use watchman from homebrew instead.

https://linear.app/comm/issue/ENG-2711

Test Plan:

nix develop

# May take a while as brew install is quite long
which watchman # Comes from homebrew install path
watchman --version # Should be 2023.01.23.00

Reviewers: atul, ashoat, varun, bartek, O1 Nix

Reviewed By: atul, ashoat, varun, O1 Nix

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6356