Enable detection of SDK version and path from environment variables (#2305)

utils/hct/hctstart.cmd

@@ -206,6 +206,9 @@ for /F "tokens=1,2*" %%A in ('%REG_QUERY% /v InstallationFolder') do (
     set kit_root=%%C
   )
 )
+if ""=="%kit_root%" (
+    set kit_root=%WIN10_SDK_PATH%
+)
 if ""=="%kit_root%" (
   echo Did not find a Windows 10 SDK installation.
   exit /b 1
@@ -228,6 +231,9 @@ for /F "tokens=1-3" %%A in ('%REG_QUERY% /v ProductVersion') do (
     )
   )
 )
+if ""=="%sdk_ver%" (
+  set sdk_ver=%WIN10_SDK_VERSION%
+)
 if ""=="%sdk_ver%" (
   echo Could not detect Windows 10 SDK version.
   exit /b 1