Test tox installation on pre-installed python

[addyess]

Overview

Allow a github job to specify its own version of python to use, and use tox from that environment to run the tests

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Details

If the github runner already has python installed via a previous step -- say:

      - name: Set up Python
        uses: actions/setup-python@v5
        with:
          python-version: '3.8'

then, this action should be able to use python-3.8 to install tox -- but it doesn't. It ignores it and uses apt to install tox.

This change tests the environment for an already installed tox or already installed pip to use for installing tox at a specific version.

• You'll also notice the tests folder expects charmcraft 2.x/stable in order to build the ubuntu charm.

[mateoflorido]

LGTM!