Fix /persist hack

hosts/nixos-testbed/configuration.nix

@@ -51,6 +51,13 @@
     };
   };
 
+  # HACK: This is here temporarily so I can use `/persist` without
+  # Impermanence.  This can be removed once impermanence is enabled.
+  # Create a group called `cfg.persistGroupName`
+  users.groups.persist = {
+    name = "persist";
+  };
+
   # HACK: This is here temporarily so I can use `/persist` without
   # Impermanence.  This can be removed once impermanence is enabled.
   systemd.tmpfiles.settings = {