Feature Request: python virtualenv naming Set via Python Platform Option or Document

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**Tell us about your request**

I would like to have a variable to create a consistent virtualenv path when running python. The current virtualenv path always (appears) to be `/var/app/venv/staging-LQM1lest` but this isn't documented. In my specific use case, I want to run cron jobs from within this virtualenv while leveraging AWS' installation of dependencies from `requirements.txt`.

**Is this request specific to an Elastic Beanstalk [platform](https://docs.aws.amazon.com/elasticbeanstalk/latest/platforms/platforms-supported.html)?**
If so, which one(s)? Python 3.7 AL2 version 3.1.5

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

The problem I'm trying to solve is having a known virtualenv location. The challenge is we are relying on an undocumented feature - that the virtualenv path will be consistently at `/var/app/venv/staging-LQM1lest`.

**Are you currently working around this issue?**

Currently we are assuming that `/var/app/venv/staging-LQM1lest` is consistently available.

**Additional context**

If the behavior was either documented in official docs or available as a "python command option" (https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/command-options-specific.html#command-options-python) such as `aws:elasticbeanstalk:environment` either would work for us.

**Attachments**
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No repository files or tests are named. Start with the Python 3.7 AL2 platform documentation and the linked Python command-options section; determine whether a stable virtualenv path can be documented or requires a new option. Done means users can reliably locate the environment without assuming `/var/app/venv/staging-LQM1lest`.

Written by the indexing model from the issue text.