ci: Install coverage with apt

Modern Python versions complain bitterly if you try to use pip from the
system installation to install additional libraries:

    error: externally-managed-environment

    × This environment is externally managed
    ╰─> To install Python packages system-wide, try apt install
        python3-xyz, where xyz is the package you are trying to
        install.

        If you wish to install a non-Debian-packaged Python package,
        create a virtual environment using python3 -m venv path/to/venv.
        Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
        sure you have python3-full installed.

        If you wish to install a non-Debian packaged Python application,
        it may be easiest to use pipx install xyz, which will manage a
        virtual environment for you. Make sure you have pipx installed.

        See /usr/share/doc/python3.11/README.venv for more information.

    note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
    hint: See PEP 668 for the detailed specification.

.github/workflows/run-tests.yml

@@ -103,7 +103,7 @@ jobs:
       - name: Run tests in container
         run: |
           docker run -v $PWD:$PWD -w $PWD --entrypoint bash ${{ env.DOCKER_IMAGE }}:build -c \
-          "pip3 install coverage && python3 -m coverage run -m unittest discover --verbose --buffer"
+          "apt-get install python3-coverage && python3 -m coverage run -m unittest discover --verbose --buffer"
 
       - name: Submit code coverage to Coveralls.io
         env: