Remove need for duplication between ct.yaml and the release GH action

Both have to know about the chart repos for the chart dependencies, so need to be kept in sync. There might be a nicer way, but https://github.com/helm/chart-releaser/issues/135 suggests maybe not.

Maybe something faintly bonkers like:
```
yq -r '.chart-repos[]' ct.yaml | sed 's/=/ /' | xargs -L helm repo add
```
in the release action is OK.