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

What's the problem this feature will solve?
I want to use pip to download linux packages onto a mac.

I know that I need to use some combination of --platform, --python-version, --only-binary=:all:. but I don't know the proper values for these options.

Describe the solution you'd like
I would like the documentation from the command line tool to be clear, or the package docum

While inspecting the installer, I noticed the fallbacks for Linux / OSX when appdirs isn't available (in data_dir()).

Wouldn't it make sense to add an if for Windows based on %APPDATA% or %LOCALAPPDATA%, before resorting to globally installing appdirs?

• brew formula
• .pkg file
• rpm package
• binary
• .exe