Only publish SharedFramework + SDK for Windows

We don't build the combined framework + SDK tarball for any Unix
platforms, but we were incorrectly trying to publish it. Make the
publish step for this artifact specific to Windows as well.

scripts/dotnet-cli-build/PublishTargets.cs

@@ -174,6 +174,7 @@ namespace Microsoft.DotNet.Cli.Build
         }
 
         [Target]
+        [BuildPlatforms(BuildPlatform.Windows)]
         public static BuildTargetResult PublishCombinedFrameworkSDKArchiveToAzure(BuildTargetContext c)
         {
             var version = CliNuGetVersion;