Fix installation of Python packages in generate docs CI job

[bbannier]

This job uses ubuntu-latest, i.e., it will automatically use the
newest version of Ubuntu available. On current versions pip install
refuses to install packages into the system since this might break
system packages. Users are expected to explicitly opt into this
potential breakage by passing the --break-system-packages flag. This
is an acceptable workaround here since we do not distribute this setup,
and currently things appear to not break.

[bbannier]

This PR fixes pip-related failures. We still run into a Sphinx build error which is addressed by zeek/zeek#3976 and should pass here once the docs have been automatically bumped.

[evantypanski]

Seems better than just failing the CI job so cool

Only nit: The requirements.txt file locks into specific versions except for pygments, should we lock that in for the rare possibility that they break a system package?

zeek-docs/requirements.txt

Line 2 in fa09ff9

Pygments>=2.12.0

Don't think it'd entirely eliminate the possibility, but may slightly reduce it

[bbannier]

Good catch with Pygments which is indeed already present in the ubuntu base image. I pushed a patch which fixes it to the exact version of the image. Should we upgrade the image I'd expect this to fail since pip install cannot remove the system package (just happened to me when I used a different version).