Tox environment is re-created each time one alternates between the `black` and `lint` environment if using Tox 4

[garrison]

Environment

• Qiskit Terra version: main
• Python version: n/a
• Operating system: Linux

What is happening?

If one uses tox4, the environment is recreated each time one switches from the black to lint environment, or vice versa.

$ tox -eblack
black: recreate env because env type changed from {'name': 'lint', 'type': 'VirtualEnvRunner'} to {'name': 'black', 'type': 'VirtualEnvRunner'}
black: remove tox env folder .tox/lint
black: install_deps> pip install -c/home/garrison/Qiskit/qiskit-terra/constraints.txt -U setuptools_rust -r /home/garrison/Qiskit/qiskit-terra/requirements.txt -r /home/garrison/Qiskit/qiskit-terra/requirements-dev.txt
[...]

$ tox -elint
lint: recreate env because env type changed from {'name': 'black', 'type': 'VirtualEnvRunner'} to {'name': 'lint', 'type': 'VirtualEnvRunner'}
lint: remove tox env folder .tox/lint
lint: install_deps> pip install -c/home/garrison/Qiskit/qiskit-terra/constraints.txt -U setuptools_rust -r /home/garrison/Qiskit/qiskit-terra/requirements.txt -r /home/garrison/Qiskit/qiskit-terra/requirements-dev.txt
[...]

How can we reproduce the issue?

See above

What should happen?

Environment should not be re-created

Any suggestions?

Several options of how to fix this:

1. Do not set both environments to the same envdir in tox.ini
2. Work around this using generative section names, in the way I did at Fix {black,lint} environment re-creation on tox4 qiskit-community/quantum-prototype-template#28. (I have not tested this since the issue I flagged in that PR was supposedly fixed in tox4.)
3. @pedrorrivero has used tox-ignore-env-name-mismatch in recent projects (e.g., pyproject-qiskit, staged-primitives) to deal with this issue.

[jakelishman]

I reckon option 1 is probably the best, but with the caveat that I don't know the precise reason we're specifically not doing that right now. I suspect it's a space-saving optimisation, but @mtreinish will know. If that's the reason, I'm probably ok with just using the extra venv storage space for people that want to use that environment - if we reduce the dependencies right down to just black[jupyter] at the right version, it probably shouldn't even be too heavy.

Option 2 I think ends up with hard-to-read code when the environments are actually separate, and the formatting env (it probably should actually include doing cargo fmt as well) probably doesn't even need to install the package (unlike the lint one), so if we're optimising we'd maybe want to do both of those. (I like the generative environments when there's only minor differences, it's just that this isn't one of those cases.)

Option 3: I'm not wild about adding a plugin that specifically calls itself out as experimental, and at the end of the day is effectively deliberately re-instating a bug into tox.