[Nix] Use vanilla direnv

Summary:
Sourcing nix_direnv seems to cause some potential
issues on machines with older versions of bash. Instead, use
a script which has some fallback behavior to determine the
desired action.

This also preserves the parent shell (e.g. zsh, fish, or bash)

Test Plan:
Tested on intel mac, using zsh and bash.

• Install direnv, e.g. brew install direnv
• Then run

# from parent directory
cd <comm root>

Reviewers: atul, ashoat, varun

Reviewed By: atul, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6236