fix(tools): venv detection use VIRTUAL_ENV environment variable (IDFGH-13545)

[nim65s]

Hi,

Here is a simple fix for #12036
This is required for Nix "venv"-like way.

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[DNedic]

Hello @nim65s , thank you for submitting this. It looks like this would fix using nix python derivations, will test.

VIRTUAL_ENV is not a perfect way to detect a virtual environment, as you could still run virtual environments scripts directly, without activating it, and it would not be set, but in this case, that should be irellevant.