fix: datastore: make relative_group_path() return relative path

previously the BackGroup trait used the datastore's
namespace_path() method to construct a base path. this would result in
it returning an absolute path equivalent to full_group_path(). use
the namspace's path() method instead to get a relative path, in-line
with backup_dir's relative_path().

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>

pbs-datastore/src/backup_info.rs

@@ -72,7 +72,7 @@ impl BackupGroup {
     }
 
     pub fn relative_group_path(&self) -> PathBuf {
-        let mut path = self.store.namespace_path(&self.ns);
+        let mut path = self.ns.path();
         path.push(self.group.ty.as_str());
         path.push(&self.group.id);
         path