Do not overwrite helm package on dependency updates

With the current CI setup, a helm package is published to the stable channel whenever Chart.yaml is changed in the main branch. That means that if only the dependency chart versions change (which happens all the time with renovatebot updating them), this will get published as the latest version, overwriting the chart version that was already there. This is undesirable, because it's changing the chart that's published under a given version number.

Perhaps we should publish to stable only on git tags, to have a more explicit release process.