Add support for building Windows x86 version of dotnet CLI

- Changes to build scripts to produce Winx86 build artifacts like
  zip/installer.
- Change to run Nuget-xplat in the same process as dotnet.exe instead of
  spinning up a new 'corerun' process.

scripts/dotnet-cli-build/PublishTargets.cs

@@ -27,6 +27,7 @@ namespace Microsoft.DotNet.Cli.Build
             // This is overkill, but I want to cover all the variables used in all OSes (including where some have the same names)
             var buildVersion = c.BuildContext.Get<BuildVersion>("BuildVersion");
             var configuration = c.BuildContext.Get<string>("Configuration");
+            var architecture = PlatformServices.Default.Runtime.RuntimeArchitecture;
             var env = new Dictionary<string, string>()
             {
                 { "RID", PlatformServices.Default.Runtime.GetRuntimeIdentifier() },
@@ -50,7 +51,8 @@ namespace Microsoft.DotNet.Cli.Build
                 { "DOTNET_CLI_VERSION", buildVersion.SimpleVersion },
                 { "DOTNET_MSI_VERSION", buildVersion.GenerateMsiVersion() },
                 { "VersionSuffix", buildVersion.VersionSuffix },
-                { "CONFIGURATION", configuration }
+                { "CONFIGURATION", configuration },
+                { "ARCHITECTURE", architecture }
             };
 
             if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))