pipenv: Ignore Python site-packages

pipenv nix wrapper contains a PYTHONPATH which has some dependencies
for pipenv installed. pipenv by default uses site-packages when
resolving packages, which means that any package that is included in
the nix wrapper is satisfied when running `pipenv install`.

But when the actual virtualenv created by pipenv is activated, it
doesn't contain those packages anymore and fails to import them.

pipenv has a flag PIP_IGNORE_INSTALLED which can be used to ignore
site-packages. Which fixes the problem of having different resolved
packages when running pipenv and when running the virtualenv.