Introduce DotNetBuild control

[mmitche]

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

This is a general identification control that essentially identifies whether the infrastructure is building in any kind of Unified Build mode. This serves as a way to conditionalize non-phase specific infrastructure in a general manner.

Generally, this is “DotNetBuildPhase != ‘’”

In general, uses of this switch should be limited to infrastructure, though it is possible that those infrastructure uses may affect the build output, especially in cases where a repo maintains a separate official build.

Should be set to true when building in UnifiedBuild mode in the orchestrator, outer build, and inner build.

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]

For dotnet/arcade#14369