Use a virtualenv for flake8 checks

Otherwise, using pip3 to install the flake8 junit.xml formatter fails
with:

    $ pip3 install flake8_formatter_junit_xml
    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.