Set working directory for JupyterHub deployment

[haobibo]

In case the image's default working dir is not set to /srv/

[manics]

Hi! Can you explain what the purpose of this change is? Z2JH is tightly coupled to the image provided in this repo, which already sets the working directory https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/645094dd64b0536aae925e92393fed7bbc6370d2/images/hub/Dockerfile#L78

This Helm chart won't work with an arbitrary hub image, anyone using a custom image is expected to closely follow the one in this repository. In the unlikely event they do have a legitmate reason for using an alternative working directory in their image this change would break it.