[Nix] Install homebrew on nix develop

Summary:
It's needed for reactotron and potentially direnv.
Since Homebrew is easy to install through the command line,
may as well automate the process if it's missing

Depends on D6311

https://linear.app/comm/issue/ENG-2752

Test Plan:
In new terminal:

# Remove /usr/local/bin from path, which is where homebrew is normally installed
export PATH=/usr/bin
# Bring in bash not from /usr/local/bin
. <(/nix/var/nix/profiles/default/bin/nix print-dev-env .#devShell)

./scripts/install_homebrew_macos.sh
# Should install homebrew, can be ran if homebrew is already installed

Reviewers: varun, atul, bartek, O1 Nix

Reviewed By: varun, atul, bartek, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6313