Fix default runtime flavor for official builds where we build native and managed in separate steps

Author: jkoritzinsky

eng/Subsets.props

@@ -84,7 +84,7 @@
   <PropertyGroup Condition="'$(RuntimeFlavor)' == ''">
     <RuntimeFlavor Condition="'$(TargetsMobile)' == 'true' and !$(_subset.Contains('+clr.nativeaotlibs+'))">Mono</RuntimeFlavor>
     <RuntimeFlavor Condition="('$(TargetsMobile)' == 'true' or '$(TargetsLinuxBionic)' == 'true') and $(_subset.Contains('+clr.nativeaotlibs+'))">CoreCLR</RuntimeFlavor>
-    <RuntimeFlavor Condition="'$(RuntimeFlavor)' == '' and ($(_subset.Contains('+mono+')) or $(_subset.Contains('+mono.runtime+'))) and (!$(_subset.Contains('+clr+')) and !$(_subset.Contains('+clr.runtime+')))">Mono</RuntimeFlavor>
+    <RuntimeFlavor Condition="'$(RuntimeFlavor)' == '' and ($(_subset.Contains('+mono+')) or $(_subset.Contains('+mono.runtime+'))) and (!$(_subset.Contains('+clr+')) and !$(_subset.Contains('+clr.runtime+') and !$(_subset.Contains('+clr.corelib+')))">Mono</RuntimeFlavor>
     <RuntimeFlavor Condition="'$(RuntimeFlavor)' == ''">$(PrimaryRuntimeFlavor)</RuntimeFlavor>
   </PropertyGroup>