pkg-python: ignore user site-packages directory

According to PEP 370 Python will also search for the packages in the
user site-packages directory. This can affect build reproducibility.

The solution is to use PYTHONNOUSERSITE=1 for all Python packages,
i.e. both host and target variants.

Fixes bug #9791.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>