Inconsistent treatment of the VIRTUAL_ENV variable

• I am on the latest Poetry version.

• I have searched the issues of this repo and believe that this is not a duplicate.

• OS version and name: macOS Mojave 10.14.2

• Poetry version: 1.0.3

• Link of a Gist with the contents of your pyproject.toml file: n/a

Issue

With VIRTUAL_ENV set, when switching into a project using a local .venv, poetry continues to use the env pointed to by VIRTUAL_ENV. However, when switching into a project using the common virtualenvs directory, VIRTUAL_ENV is ignored and the project virtualenv is used.

Switching from project using common virtualenvs location to one using local .venv while VIRTUAL_ENV is set - notice output of poetry env info -p is incorrect (or more specifically, reflects VIRTUAL_ENV):

npyscreen$ echo `poetry env info -p`
/Users/ryan/Library/Caches/pypoetry/virtualenvs/npyscreen-KP5NaiQl-py3.8

npyscreen$ export VIRTUAL_ENV=/Users/ryan/Library/Caches/pypoetry/virtualenvs/npyscreen-KP5NaiQl-py3.8

npyscreen$ cd ../testproj/

testproj$ echo `poetry env info -p`
/Users/ryan/Library/Caches/pypoetry/virtualenvs/npyscreen-KP5NaiQl-py3.8

testproj$ unset VIRTUAL_ENV && echo `poetry env info -p`
/Users/ryan/Documents/workspace/testproj/.venv

Switching from project using local .venv to one using common virtualenvs dir - output of poetry env info -p reflects project setting despite VIRTUAL_ENV:

testproj$ export VIRTUAL_ENV=/Users/ryan/Documents/workspace/testproj/.venv

testproj$ cd ../npyscreen

npyscreen$ echo `poetry env info -p`
/Users/ryan/Library/Caches/pypoetry/virtualenvs/npyscreen-KP5NaiQl-py3.8

This is due to code starting at:
https://github.com/python-poetry/poetry/blob/12db4a5e189ea8514e70f22279ae540274a0e5af/poetry/utils/env.py#L303
which sets env for the common directory virtual envs but not the local .venv envs, which is then checked a few lines later at:
https://github.com/python-poetry/poetry/blob/12db4a5e189ea8514e70f22279ae540274a0e5af/poetry/utils/env.py#L322

I have not attempted a fix/PR (though it should be simple) as I am not knowledgeable on the reasons the code might have been written this way, or what other (negative?) effects a change here might induce.

Start in poetry/utils/env.py at the logic around the referenced lines 303 and 322, then reproduce the two project-switching scenarios with VIRTUAL_ENV set and unset. Confirm the intended behavior for local .venv and common-directory environments, and add or update coverage so both cases treat VIRTUAL_ENV consistently.