fix(storage): never write blob links when FS mirroring is disabled

This MR prevents filesystem metadata from being written when mirroring is disabled when cross repository blob mounting.

Previously, the logic to disable layer blob linking was only contained within the blobWriter which links the layer metadata after a blob is successfully uploaded. However, blob links are also written during when a cross repository blob mount succeeds.

To prevent this, we now disable blob linking directly in the linked blob store, while allowing it to continue to write to the common blob storage.