[Nix] Add localstack cli to dev shell

Summary:
Add localstack cli to 'nix develop' shell.

Depends on D5243

Test Plan:

nix develop
which localstack # should point to nix store
localstack --help # should print usage

Reviewers: abosh, varun, atul, O1 Nix

Reviewed By: varun, atul, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5244