[keyserver] Specify MariaDB port in backups code

Summary:
In D12336 we added support to DBConfig for a port, but we forgot to update the backups code to specify this port when calling mysqldump.

This addresses at least part of ENG-9077.

Test Plan: Ran this on my keyserver instance running in a Docker container on my personal server and confirmed that it works

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13134