[Nix] Add more support for localstack

Summary:
Improve localstack experience. The aws sdk crate
would fail unless you had aws credentials. This allows for
tunnelbroker to be ran with just localstack and "nix develop"

Part of: https://linear.app/comm/issue/ENG-3669

Test Plan:

nix develop
comm-dev services start

cd services/terraform
./run.sh
awslocal dynamodb list-tables

Reviewers: bartek, varun, ashoat, atul, tomek

Reviewed By: varun, ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7767