python-native: disable user site support

The user site-packages gets inserted into sys.path ahead of the system site
directories, so a site package installed there will be used in preference to
what's in our sysroot, causing less deterministic builds, and potential build
breakage, depending on what the user has installed there. Disable it for our
native python, so they don't affect our builds.

JIRA: SB-2579

Related to SB-2073.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #127 from kergoth/sb-2579

python-native: disable user site support