build: simplify the SDK packaging step

The swift install rules now can distribute the auxiliary files which
removes the dependency on the source tree.  Update the rules
accordingly.

Author: compnerd

utils/build-windows-toolchain.bat

@@ -940,8 +940,7 @@ msbuild %SourceRoot%\swift-installer-scripts\platforms\Windows\sdk.wixproj ^
   -p:OutputPath=%PackageRoot%\sdk\ ^
   -p:IntermediateOutputPath=%PackageRoot%\sdk\ ^
   -p:PLATFORM_ROOT=%PlatformRoot%\ ^
-  -p:SDK_ROOT=%SDKInstallRoot%\ ^
-  -p:SWIFT_SOURCE_DIR=%SourceRoot%\swift\
+  -p:SDK_ROOT=%SDKInstallRoot%\
 :: TODO(compnerd) actually perform the code-signing
 :: signtool sign /f Apple_CodeSign.pfx /p Apple_CodeSign_Password /tr http://timestamp.digicert.com /fd sha256 %PackageRoot%\sdk\sdk.msi