Fix examples/submission-stress-test

[tcompa]

server-config | ERROR:root:Server returned 422
server-config | ERROR:root:Original request: POST http://slurm:8000/api/v2/task/collect/pip/
server-config | ERROR:root:Original payload: {"package": "/fractal_tasks_mock-0.0.1-py3-none-any.whl"}
server-config | ERROR:root:Server error message: {'detail': "User settings are not valid for FRACTAL_RUNNER_BACKEND='slurm'. Original error: 2 validation errors for SlurmSudoUserSettings\nslurm_user\n none is not an allowed value (type=type_error.none.not_allowed)\ncache_dir\n none is not an allowed value (type=type_error.none.not_allowed)"}