Adding HOME envvar to venv creation and install

See https://github.com/pypa/virtualenv/issues/535 as well as
https://github.com/poise/python/pull/72 for the reasons why.  

The underlying bug seems to be described at 
https://bitbucket.org/pypa/distlib/issue/39/using-distlib-with-pip-fails-when-user-has

Hopefully they will fix it and this will become unnecessary.