ci: use apt instead of pip to install tox

[james-garner-canonical]

Description

Github's rollout of 24.04 for ubuntu-latest has now reached python-libjuju, so pip install tox no longer works -- 24.04 prevents installing packages using pip.

This PR switches all our run: pip install tox lines in github actions to run: sudo apt install tox.

QA Steps

Tests no longer fail when trying to install tox, as they started doing in the last 12 hours.

[james-garner-canonical]

Oh looks like this is the wrong approach as the attempt to install tox happens in an earlier step anyway.
test.yaml

      - name: Setup operator environment
        uses: charmed-kubernetes/actions-operator@main
        with:
          provider: lxd
          juju-channel: 3.4/stable

Test output

Run charmed-kubernetes/actions-operator@main
Install core snap
Install LXD
Initialize LXD
Install tox
  /usr/bin/which tox
  /usr/bin/which pip
  /usr/bin/pip
  pip is available, install tox
  /usr/bin/pip install tox
  error: externally-managed-environment

So we can probably remove our manual installation of tox altogether. And the correct fix seems to be running our setup python step before running setup operator environment -- see here.