Introduce DotNetBuildOrchestrator control

[mmitche]

See https://microsoft-my.sharepoint.com/:w:/p/mmitche/EdchH8A4okpOoKhkki1sJEYB0LyAaaCyX5_6A887gL744Q?e=JERvht for full design:

When “true”, indicates that the infrastructure is executing within the orchestrator, outer repo build, and inner repo build.
This is roughly equivalent to DotNetBuildFromSourceFlavor as “Product” in the current control set.

Should be active when in the orchestrator or any sub builds (if invoked via the orchestatrator)

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mmitche]

Will be completed with dotnet/installer#18306

[mmitche]

Changes complete