Don't pass -os twice for windows in DotNetBuild.props (#114732)

Author: akoeplinger

eng/DotNetBuild.props

@@ -32,7 +32,7 @@
 
       <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)arch $(TargetArchitecture)</InnerBuildArgs>
       <InnerBuildArgs Condition="'$(_portableOS)' == 'win'">$(InnerBuildArgs) $(FlagParameterPrefix)os windows</InnerBuildArgs>
-      <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)os $(_portableOS)</InnerBuildArgs>
+      <InnerBuildArgs Condition="'$(_portableOS)' != 'win'">$(InnerBuildArgs) $(FlagParameterPrefix)os $(_portableOS)</InnerBuildArgs>
       <!-- Mobile builds are never "cross" builds as they don't have a rootfs-based filesystem build. -->
       <InnerBuildArgs Condition="'$(CrossBuild)' == 'true' or ('$(TargetArchitecture)' != '$(BuildArchitecture)' and '$(TargetsMobile)' != 'true')">$(InnerBuildArgs) $(FlagParameterPrefix)cross</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)configuration $(Configuration)</InnerBuildArgs>