migrator: Instantiate store factory

Created by: efritz

This is a refactor step towards #39578 (closed). This PR passes a concrete value (placeholder value was nil) for the store factory in the migrator run-out-of-band-migrations command. This ensures that when we have both the migration runner and out of band migration runner stores initialized, they will not try to connect to the database independently. This causes issues with us registering prometheus metrics in the dbconn package.

Test plan

Existing CI pipelines.