Use RuntimeOS instead of OSGroup for defaulting PackageRID

Author: weshaggard

pkg/Microsoft.Private.CoreFx.NETCoreApp/netcoreapp.rids.props

@@ -1,9 +1,9 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup Condition="'$(PortableBuild)' == 'true'">
-    <PackageRID>win-$(ArchGroup)</PackageRID>
-    <PackageRID Condition="'$(OSGroup)' == 'OSX'">osx-$(ArchGroup)</PackageRID>
-    <PackageRID Condition="'$(OSGroup)' == 'Linux'">linux-$(ArchGroup)</PackageRID>
+    <PackageRID>linux-$(ArchGroup)</PackageRID>
+    <PackageRID Condition="'$(_runtimeOSFamily)' == 'osx'">osx-$(ArchGroup)</PackageRID>
+    <PackageRID Condition="'$(_runtimeOSFamily)' == 'win'">win-$(ArchGroup)</PackageRID>
   </PropertyGroup>
   <PropertyGroup Condition="'$(PortableBuild)' != 'true'">
     <PackageRID>$(RuntimeOS)-$(ArchGroup)</PackageRID>