Set BUILD_ARCH to x64 by default if none is specified (#4510)

Author: hekota

utils/hct/hcttest.cmd

@@ -1,10 +1,16 @@
 @echo off
 setlocal ENABLEDELAYEDEXPANSION ENABLEEXTENSIONS
 
+rem Default build config is Debug
 if "%BUILD_CONFIG%"=="" (
   set BUILD_CONFIG=Debug
 )
 
+rem Default build arch is x64
+if "%BUILD_ARCH%"=="" (
+  set BUILD_ARCH=x64
+)
+
 rem Whether we built the project using ninja as the generator.
 set GENERATOR_NINJA=0