Use venv in CI

[stephanie-eng]

Description

Fix this error:

 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/python3.12/README.venv for more information.

This should make CI pass the "Install Python Dependencies" step.

[sea-bass]

You may need to add a note to the htmlproofer.sh instructions that this needs to be run in a virtual environment. Otherwise, I think we should go for it and fix the actual build errors separately.

[stephanie-eng]

I updated a doc page and added in a comment. It looks like that doc page I updated has other issues (the htmlproofer script requires ruby to be installed, the requirements currently say Galactic), but those changes can be done in separate PRs - those issues are not particularly related to the virtual environment stuff.