[nix][scripts] Rename localstack_main to localstack-main

Summary: When we updated localstack in ENG-5724, we pulled in this change. As a result comm-dev services stop stopped working because we had the wrong name for the Docker container.

Test Plan:

1. Confirm that comm-dev services stop no longer prints the error "Error response from daemon: No such container: localstack_main"
2. Confirm that comm-dev services start following comm-dev services stop no longer prints the message "localstack is already running, skipping localstack initialization"

Reviewers: bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11550