Introduce DotNetBuildPhase control

[mmitche]

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

Exclusive phase control identifying the phase currently executing.
Generally, this replaces uses of ArcadeInnerBuildFromSource (exclusive inner build) as well as common conditionals like “ArcadeBuildFromSource && !ArcadeInnerBuildFromSource”.

This is active in all levels of the 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]

Complete for Repo and InnerRepo phases.

[mmitche]

This is completed when dotnet/installer#18306 goes in.