Ruby version picking is broken

[andreyvit]

With the transition to the actions-based build, the old Ruby version picking UI is totally broken.

Ideally, we want to:

• Port the Rubies preferences pane from LR3 code
• Use Package Manager Kit to populate the list of available rubies
• Respect the newly selected Ruby version when building

But we should also consider if we still need to pick the Ruby version to build with, or if we can maybe get away without it for a couple of releases. (Maybe need to add analytics on how often a non-system Ruby gets picked?)