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
so hopefully they will fix it and this will become unnecessary.