It is quick and easy to use Hyper-V Quick Create to get an Ubuntu virtual machine running on a Windows 10 computer. However, if this method is used, you may end up with a tiny Ubuntu virtual disk that will not be useful for any serious work and it is less obvious than the initial setup how to increase the size of this disk.
These steps fix the problem:
Say you want to modify each project reference once you know what platforms it can build as, the only place to do it (before PlatformNegotiation kicks in) is...
BeforeTargets=_GetProjectReferencePlatformProperties
AfterTargets=_GetProjectReferenceTargetFrameworkProperties
_GetProjectReferenceTargetFrameworkProperties is the target that calls MSBuild on a project reference to gather its information (and see what it would build as). Only AFTER this target you can mess with ProjectReferences while understanding what they can build as.
If you target runs between the two above, you should have an _MSBuildProjectReferenceExistent item you can mess around with.