"conda activate" does not set PYTHONOUSERSITE

[pitrou]

When "conda activate" is called to activate an environment, it doesn't set the PYTHONNOUSERSITE environment variable. As a result, Python packages installed into the "user site" directory (e.g. $HOME/.local/lib/python2.7/site-packages) are visible inside the environment. This can have severe consequences for the user.

[nehaljwani]

xref: conda/conda#7173