[backup] Standardize localstack config
Summary: Based on the blob diff for the same thing: D8835
Test Plan: cargo run and cargo run -- --localstack_endpoint 'testing' - check if logs show the correct endpoint
Reviewers: bartek, kamil, patryk, jon
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D8869