Update build.cmd to match latest template

- aspnet/Universe#347
- `%KOREBUILD_VERSION%` doesn't work without this fix

Author: dougbu

build.cmd

@@ -28,12 +28,11 @@ IF NOT EXIST %NUGET_PATH% (
     copy %CACHED_NUGET% %NUGET_PATH% > nul
 )
 
+SET KOREBUILD_DOWNLOAD_ARGS=
+IF NOT "%KOREBUILD_VERSION%"=="" (
+    SET KOREBUILD_DOWNLOAD_ARGS=-version %KOREBUILD_VERSION%
+)
 IF NOT EXIST %KOREBUILD_FOLDER% (
-    SET KOREBUILD_DOWNLOAD_ARGS=
-    IF NOT "%KOREBUILD_VERSION%"=="" (
-        SET KOREBUILD_DOWNLOAD_ARGS=-version %KOREBUILD_VERSION%
-    )
-    
     %BUILD_FOLDER%\nuget.exe install KoreBuild-dotnet -ExcludeVersion -o %BUILD_FOLDER% -nocache -pre %KOREBUILD_DOWNLOAD_ARGS%
 )