consider dropping venv iteration

[jonapich]

coveo-stew was designed to be able to prepare multiple venvs and then test them in one go:

# create 3 envs
poetry env use python3.9
poetry env use python3.10
poetry env use python3.11

# test them all
stew ci

I can only think of downsides:
- execution takes Nx the time (as much venvs you got)
- user doesn't always notice that stew ci runs twice
- when projects evolve and change their python constraint, old environments are used and fail
- the user isn't aware of this functionality and only learns it when a problem occurs

Anyway, most CI wants to run supported versions in parallel, so they wouldn't use this.

We should instead just test the active one (i.e.: poetry env list displays which one is current).