No support for SCOPE_PREFIX in logical-backup, which *is* supported in WAL backups

[eigood]

https://github.com/zalando/postgres-operator/blob/a081173f26f2d6b7dd6408d7cee8b307fd8e6578/docker/logical-backup/dump.sh#L100

I've set a SCOPE_PREFIX in my operator deployment to "${machine_name}/", so that the path in the bucket is "/spilo/${machine_name}/${postgres_name}/${guid}". However, the logical backups are stored into "/spilo/${postgres_name}/${guid}". The PREFIX is set via terraform, to the name of the cluster being created(inside hetzner, but that doesn't matter for this issue). Having a per-cluster prefix lets me clear out all backups when the cluster is being purged(for developer instances), instead of having to use lots and lots of per-developer buckets.