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>