jupyter: Create jupyter user as system user

Before, with services.jupyter.enable set but no user specified, user creation
failed with:
```
error:
Failed assertions:
- Exactly one of users.users.jupyter.isSystemUser and users.users.jupyter.isNormalUser must be set.
```

Set isSystemUser to true when creating the jupyter user.

nixos/modules/services/development/jupyter/default.nix

@@ -196,6 +196,7 @@ in {
         extraGroups = [ cfg.group ];
         home = "/var/lib/jupyter";
         createHome = true;
+        isSystemUser = true;
         useDefaultShell = true; # needed so that the user can start a terminal.
       };
     })