Adds ability to append `-unstable` to released chart

• When sending the environment variable UNSTABLE==true into a pipeline, we'll utilize this as a signal that the chart to be released is in an unstable format
• This creates a new job dedicated to unstable projects
• We'll create a new repository under s3://<bucket>/unstable
• This also downloads the index file in that location to add it to the artifacts for our CI jobs

Addresses: gitlab-com/gl-infra/delivery#624 (closed)