Issue description

When the environment variable PIPENV_SITE_PACKAGES is set, pipenv always recreate new virtualenv on each invoke of pipenv even if it is not a pipenv install command.

Expected result

• pipenv run or pipenv shell should use existing virtualenv instead of creating a new one.
• PIPENV_SITE_PACKAGES should only affect pipenv install command.

Actual

i've got a bunch of Pipenv shell buffers and don't know which is which. It would be handy to name them when you create them, like I can do with ^U M-x shell. (I tend to name my shell buffers for the dir I'm in working in to help me remember).

Thanks for pipenv.el.

Albo oauth, nie chcę trzymać tutaj haseł

https://github.com/thoth-station/python/blob/eda4af0686f438baf210913670e713172035f59a/thoth/python/source.py#L167-L172

This issue was generated by todo based on a TODO comment in eda4af0686f438baf210913670e713172035f59a when #3 was merged. cc @fridex.