[Nix] Only prompt for direnv usage on install

Summary:
It's actually quite hard to determine if a shell is interactive
between difference shells and environments.

Move the prompt to use direnv to the install_nix.sh script to condense
all possible prompt interactions into a single script.

https://linear.app/comm/issue/ENG-2847

Test Plan:

rm -rf ~/Library/Caches/app.comm/install-direnv
brew uninstall direnv

./scripts/install_nix.sh

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

Reviewed By: ashoat, varun, O1 Nix

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6532