Create an unstable helm repository for auto-deploy

In order to not clutter our existing helm stable repository, let's create a repo for which auto-deploy tags can publish their charts too.

Proposed plan of action for this issue:

1. Modify charts/gitlab's trigger to determine if the tag is an auto-deploy tag - gitlab-org/charts/gitlab!1180 (merged)
   • if yes, set UNSTABLE to true
   • if no, don't bother setting UNSTABLE
2. Modify charts.gitlab.io's release_chart job to check for UNSTABLE - charts/charts.gitlab.io!143 (merged)
   • if UNSTABLE == "true"
     • rename our chart to gitlab-unstable during clone