Update Poetry to use VIRTUALENVS_HOME for location of virtualenvs

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Feature Request

Update Poetry to use the VIRTUALENVS_HOME environment variable as its first choice for the location to store virtual envs.

This is a very simple change request, but it has many positive ramifications for developers who tend to use a wide variety of tools to develop Python solutions.

Currently Poetry provides a config setting and a Poetry specific environment variable. Neither of these is portable across the many tools used by developers that interact with virtual environments.

My justification is that currently there are dozens of tools for python that interact with virtualenvs in some way. They currently all store virtual envs in their own custom locations, which varies by platform. If they provide an option to change the location used for storing venvs it is with a custom environment variable for that tool. Given the current fragmentation of the python developer tool-set for managing environments, packaging, etc. I am trying to start small with one small change that could help all developers.

Start by tracing how Poetry currently selects the virtualenv storage location and how its existing configuration and environment variable are handled. Confirm the precedence for VIRTUALENVS_HOME, add or update coverage for that behavior, and verify that the existing configuration remains supported.