Decide how stable builds should get turned on when it comes time to ship GA or a preview.

[mmitche]

We need to come up with a mechanism for which this happens. Some key questions:

• Should this happen at a repo level or at the orchestrator level?
• Should this behavior flow back to the isolated repos?

[mmitche]

We should turn this on by setting DotNetFinalVersionKind=prerelease in the VMR root. This will be documented in dotnet/dotnet#1043