[VMR] [PoC] Do not pass DotNetBuildFromSource when building VMR PoCs

[mmitche]

When building the VMR as Microsoft, we want to a produce a superset of outputs, and allow a superset of inputs from what would be produced out of the Linux build that our distro partners would use.

Right now the VMR build infra passes DotNetBuildFromSource. We need to add a way to not pass this switch. However, some places where this switch is used will be required for the VMR vertical build.

For the VMR PoCs, we should not be passing this switch. The use of DotNetBuildFromSource will not give a full accounting of the PoCs. Instead, we should be passing another placeholder switch (e.g. DotNetBuildVertical), and using this in places where we need for the PoCs.

[dotnet-issue-labeler]

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]

Related to #2974

[akoeplinger]

I also found that we always pass DotNetBuildFromSourceFlavor=Product in the VMR, but we have several checks for != Product in the repos.

Related properties that came up are ArcadeBuildFromSource, ArcadeInnerBuildFromSource and ExcludeFromSourceBuild.

[akoeplinger]

Learned that DotNetBuildFromSourceFlavor=Product is supposed to mean building the whole product (e.g. the source-build/VMR), but individual repos can have CI jobs without that property when building only their slice for validation.

[ViktorHofer]

This is now implemented for the Windows VMR build.

[ViktorHofer]

Related: #3965

[ViktorHofer]

Closing as completed.