Unable to build upstream roslyn repository using a source-built SDK

We are trying to build and test the roslyn repo (standalone) on a ppc64le machine to verify that it's fully working and all known bugs have been fixed. On that architecture, the only SDK that's generally available for users is a source-build SDK. Unfortunately, roslyn has a number of projects that need the `Microsoft.NET.Sdk.WindowsDesktop` sdk. So a source-build SDK can't actually build or test roslyn.

From what I can tell, the errors are all in projects that are not used in source-build. The projects are (eventually) marked as `ExcludeFromSourceBuild=true`. When source-build is building roslyn, it uses arcade overrides to feed it an empty SDK to get those projects to restore (but not build). But that approach of overriding arcade isn't really available when we want to build/test roslyn standalone.

Is there some way to use a source-build SDK to build+test roslyn standalone?