espidf installer: Make sure python does not come from virtual env

[N3xed]

In the esp-idf installer when it tries to create a python virtual env with

python esp-idf-v4.3.1/tools/idf_tools.py --idf-path esp-idf-v4.3.1 --quiet --non-interactive install-python-env

if python is already from a virtual env this installation fails with:

ERROR: This script was called from a virtual environment, can not create a virtual environment again

Desired state:
Detect this and try to get the actual python executable from which the virtual env was created.