Add default value for `lfs.store_path` option in the NixOS module #4641

Since it's mandatory to use Git LFS, and since other files already have a default path (/var/lib/knot/{repos,sealed-keys,ssh_host_key}), it would make sense to set the LFS store path to /var/lib/knot/lfs. Is it not set to a default value intentionally, or it was just an oversight when adding the option?