CI should fail if a tox env doesn't exist

[sed-i]

tox will happily pass for non-existing testenvs:

$ tox -e volkswagen
  volkswagen: OK (0.84 seconds)
  congratulations :) (0.87 seconds)

This is especially misleading when instead of unit we have unit-k8s and unit-machine.

We should probably change the CI command to run tox to:

if tox -l | grep -q '^unit$'; then tox -e unit; else echo "Error: 'unit' testenv does not exist"; fi

[lucabello]

Funnily enough, it seems like this was solved upstream: tox-dev/tox#3089

The tox 4.9.0 release https://github.com/tox-dev/tox/releases/tag/4.9.0 includes that change. Our CI is using the latest tox version, so nothing to do here!