Use PIP_INDEX_URL with pipenv

[gardar]

pipenv does not read the PIP_INDEX_URL environment variable (see: pypa/pipenv#4808 ) so to when using a local pip mirror I have to define the mirror url twice, both with PIP_INDEX_URL env var (for installing pipenv) and also in the Pipfile.
While it's not a huge problem it would still be nice to be able to configure the mirror with a single env var.

I see two possible solutions:

To either pass the value of PIP_INDEX_URL to the pipenv install command with the --index option

pipenv install --deploy --index $PIP_INDEX_URL

or write the PIP_INDEX_URL env var to a pip.conf or pip.ini during the assemble.

[frenzymadness]

@hrnciar this is something you might be interested in. Seems like an easy fix.