[build-windows-toolchain.bat] Fix environment variable name

Author: AnthonyLatsis

utils/build-windows-toolchain.bat

@@ -81,7 +81,7 @@ call :CloneRepositories || (exit /b 1)
 
 :: We only have write access to BuildRoot, so use that as the image root.
 powershell.exe -ExecutionPolicy RemoteSigned -File %~dp0build.ps1 ^
-  %HostArchName% ^
+  %HostArchNameArg% ^
   -SourceCache %SourceRoot% ^
   -BinaryCache %BuildRoot% ^
   -ImageRoot %BuildRoot% ^