Remove temporary directory after a run finished

It would be a nice addition, if a `cml runner launch` would have an option to automatically cleanup the working directory after a run finished.

https://github.com/iterative/cml/blob/5e9fcd26e9683db26a429eb5f34b989134694d4b/bin/cml/runner/launch.js#L109

```bash
cml runner launch --cleanup-workdir
```