ci: fetch stable/dev releases using helm show to avoid cache issues
Previously we fetched the latest stable and dev release from metadata we
have maintained, but doing that and then using helm install of that
version made us use two sources of data, where one could be relatively
outdated due to hitting a cache while the other may not.
Using helm show, we rely on the helm chart repository website
returning a index.yaml listing versions etc both when checking whats the
latest versions, and when installing - like that we avoid a cache issue
I think.
Sibling PR to https://github.com/jupyterhub/zero-to-jupyterhub-k8s/pull/3256