[backup] Make blob url configurable with env var

Summary:
Matches the config in the reports service. We need to be able to set url for the blob service from terraform.

Depends on D9064

Test Plan: Try running it with the env var, check if it works. Tested on staging with env var set by terraform.

Reviewers: bartek, patryk, kamil, jon

Reviewed By: bartek, jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9065