epilogue not working

### Bug description

It seems that epilogue is not working after stopping the notebook. I set up the `epilogue` parameter in the configuration file like this:

`c.SlurmSpawner.req_epilogue = """!/bin/bash echo 'hostname'"""` (just an example). I am able to see the command in jupyterhub log after `cmd` but nothing happens.

#### Expected behaviour

run epilogue after stopping the notebook.

#### Actual behaviour

### Your personal set up

- OS: CentOS 8

- Version(s):
* jupyterhub 1.5.0
* batchspawner 1.1.0

```python
# jupyterhub_config.py
c.SlurmSpawner.req_epilogue = """!/bin/bash echo 'hostname'"""
```