Option to override pip's --exists-action, or PIP_EXISTS_ACTION

[simion]

Hello,

I'm trying to find a way to specify --exists-action for pip install command, but it seems there's no way to override it.

Here: https://github.com/pypa/pipenv/blob/master/pipenv/core.py#L1506 you force "PIP_EXISTS_ACTION": vistir.misc.fs_str("w")

I need a way to set either PIP_EXISTS_ACTION=i or --exists-action=i (ignore a package if it's already installed)
This would make installation really really (really) fast, if the virtual environment exists.

We need this because we cache and restore .venv, and this option would make CI much faster :)

Thanks

[techalchemy]

Thanks for the suggestion and for the accompanying PR, I will toggle this a bit myself but I don't really see why we shouldn't do this... /cc @uranusjr @frostming