build.ps1: allow building without the legacy SDK

Update the installer build to exclude the legacy SDK content from
packaging. This prepares us to remove the legacy SDK entirely from the
build.

Author: compnerd

utils/build.ps1

@@ -3902,6 +3902,7 @@ function Build-Installer([Hashtable] $Platform) {
   $Properties = @{
     BundleFlavor = "offline";
     ImageRoot = "$(Get-InstallDir $Platform)\";
+    IncludeLegacySDK = if ($HostPlatform.DefaultSDK -match "Experimental") { "False" } else { "True" };
     INCLUDE_SWIFT_DOCC = $INCLUDE_SWIFT_DOCC;
     SWIFT_DOCC_BUILD = "$(Get-ProjectBinaryCache $HostPlatform DocC)\release";
     SWIFT_DOCC_RENDER_ARTIFACT_ROOT = "${SourceCache}\swift-docc-render-artifact";