Fix system site packages detection default (#11956)
See https://docs.python.org/3/library/site.html and
https://github.com/python/cpython/blob/a549f439384b4509b25639337ffea21c2e55d452/Lib/site.py#L614-L639:
The default is for system site packages is true, they have to be
disabled by setting the key to something that is not true.
