[Nix] Add dev shell to .envrc

Summary: Direnv should reload when we change the dev shell as well.

Test Plan: Could patch in another diff (e.g. D6356) and assert that direnv reloaded.

Reviewers: atul, varun, bartek, ashoat

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6392