[Nix] Enable localstack

Summary:
Make localstack the default target when interacting with
AWS in the nix development environment.

Test Plan:

which localstack # points to nix store

This gets tested more thoroughly in a later diff.

Reviewers: ashoat, bartek, varun, O1 Nix

Reviewed By: bartek, varun, O1 Nix

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6787