building platforms package (#42897)

Author: Anipik

src/packages.builds

@@ -24,6 +24,9 @@
       <AdditionalProperties>$(AdditionalProperties)</AdditionalProperties>
     </Project>
     <!-- add specific builds / pkgproj's here to include in servicing builds -->
+    <Project Include="$(MSBuildThisFileDirectory)..\pkg\Microsoft.NETCore.Platforms\Microsoft.NETCore.Platforms.builds">
+      <AdditionalProperties>$(AdditionalProperties)</AdditionalProperties>
+    </Project>
   </ItemGroup>
 
   <!-- Need the PackageIndexFile file property from baseline.props -->