allow using --system when adding packages interactively

[doubledgedboard]

Is your feature request related to a problem? Please describe.

It's frustrating using pipenv inside a vscode dev container. There's no reason to use virtualenvs inside docker, but in dev containers, you're doing interactive adding of or managing of Pipfile entries, and it's non-trivial to generate those entries with something other than pipenv

Describe the solution you'd like

I'd like to be able to use pipenv install --system foo and have it generate Pipfile and Pipfile.lock and install into system directory

Describe alternatives you've considered

Currently I have to do a really ugly workaround of temporarily installing it into a virtualenv inside the dev container, which updates the Pipfile, and then I can run pipenv install --system to actually put it into the container's python.

Then I manually delete the virtualenv it created. It's pointless extra work.

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I use pipenv on a machine / network that I cannot export the output of pipenv support from