Update vcbuild.bat

The PYTHON environment variable should be quoted in case there's spaces in the path. For example assume `set PYTHON=C:\some path\python.exe` you'd want the command to be `"C:\some path\python.exe"` so PYTHON would need to be expanded in quotes `"%PYTHON%"`. This is typical in Windows because environment variables usually don't contain quotes and instead you're expected to quote on expansion.