When restoring via Build.proj the argument nodeReuse is not respected outside CI. Outside of CI the build defaults to using a quiet restore and hence shells out to MSBuild vs. using the MSBuild task. In doing so the nodeReuse argument originally passed to MSBuild is lost.

    <Exec Command='$(_MSBuildCmd) "@(ProjectToBuild)" /nologo /m /v:quiet /t:Restore $(_SolutionBuildPropsCmdLine) /p:__BuildPhase=SolutionRestore'
          Condition="'$(RestoreUsingNuGetTargets)' != 'true' and '$(Restore)' == 'true' and '$(_QuietRestore)' == 'true'" StandardOutputImportance="normal" />

This is very problematic for Roslyn. In our bootstrap phase we pass the property $(BootstrapBuildPath) around to change where the C# compiler is loaded from. The restore phase does not propogate this property nor does it propogate nodeReuse. Hence the first invocation of MSBuild for restore uses the standard C# compiler path, it hangs around and our subsequent invocation can't override as the C# task has already been loaded and our bootstrap is broken.