Issue

tox -e env1 -e env2 runs the two environments in tox 3.x but runs only env2 in tox 4.x.

I don't know if that change is intentional but it's sure to break quite a few CI that rely on -e being additive rather than overriding a former value. I would suggest to either fix the behaviour to be consistent with 3.x or put some strong warning in the changelog and documentation that one needs to use the comma separated syntax: tox -e env1,env2

Environment

Provide at least:

• OS: Debian Linux (unstable)

$ tox --version
4.12.0 from /usr/lib/python3/dist-packages/tox/__init__.py

Thank you for maintaining tox! It's a great tool.