Migrate existing update mechanism to new conda-based installers

[mrclary]

The existing update plugin needs to be updated for the new conda-based installers.
See this comment.

1. Migrate the update mechanism as is; do not consider inplace conda updates, just installer download and execution.
2. Modify to trigger download of installer for some releases and trigger inplace conda updates for other releases.
   • What releases should trigger a new installer download and which should trigger a conda update (update inplace)?
   • How should the runtime conda environment be structured/updated?

Should this be split across multiple milestones?