Add SourceBuildManagedOnly to SourceBuild.props (#6507)

Fixes dotnet/source-build#2068

Context
This was something I discovered while trying to consume the Source-Build intermediate package produced by the first ArPow changes made in #6387

Changes Made
Set the SourceBuildManagedOnly property correctly.

How Tested
Verified the resulting Microsoft.SourceBuild.Intermediate.msbuild package is not named rid specific.

Author: MichaelSimons

eng/SourceBuild.props

@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <GitHubRepositoryName>msbuild</GitHubRepositoryName>
+    <SourceBuildManagedOnly>true</SourceBuildManagedOnly>
   </PropertyGroup>
 
   <Target Name="ConfigureInnerBuildArgs" BeforeTargets="GetSourceBuildCommandConfiguration">