Update dogfood to use insiders helm chart

Created by: caugustus-sourcegraph

Followup to https://github.com/sourcegraph/deploy-sourcegraph-dogfood-k8s/pull/4317: right now, k8s.sgdev.org tracks the official helm chart releases. We should instead track the insiders builds so we can test pre-release changes.

Unanswered questions:

• Will renovate manage the automatic version updating, or some other process?
• Do we need to be able to run a diff between the old and new versions?