fix (loki): guard monolith local storage config with minio enablement check

General MR

Summary

This MR fixes a startup issue in Loki when deployed in monolith mode with minio enabled.

I've verified that this change causes no issues with Loki in other deployment modes or when minio is disabled in monolith mode.

Relevant logs/screenshots

Proof of activity in local minio pool:

Proof of network activity from loki to minio:

Linked Issue

Closes big-bang/product/packages/loki#196 (closed)

Upgrade Notices

N/A