Pre-create backups dir to support volume/bind mount.

[mwijngaard]

When running Fuseki in something like Kubernetes, it's quite hard to work with backups since Fuseki doesn't expose them over http. This change pre-creates the backups dir so permissions get set that allow mounting a volume, similar to what happens with the databases dir.

By mounting a volume, you:

1. Make backups persistent
2. Allow the backups to be shared with a sidecar container that exposes it over the network (I use sftp, but webdav could work just as well).