Reproducer for tox/virtualenv installing the wrong pip

Build the docker image that uses pyenv to install python 3.7 and 3.13

docker build --tag tox-pip-reporoducer --progress plain .

Now launch a container from the image and run the script that runs tox (cleaning up .tox before each run) with 3 different configurations (tox1.ini - tox3.ini). All of them have py37 and py313 factors. The py37 ones are the problem here: Configuration 1 and 3 will fail. It appears that pip 24.2 from python 3.13 is installed and used in the tox virtual environment. That's a problem because with 24.1 pip dropped support for python 3.7.

export USER_ID=$(id -u)
export GROUP_ID=$(id -g)

```bash
docker run -it --rm -v ./:/demo -v ./out:/demo/out -w /demo -u "$USER_ID:$GROUP_ID" tox-pip-reporoducer ./run.sh

You can find the output of the 3 tox runs in the out directory.