Fix installation of Python packages in generate docs CI job

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.

.github/workflows/generate-docs.yml

@@ -18,6 +18,6 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - name: Fetch Dependencies
-        run: sudo pip3 install -r requirements.txt
+        run: sudo pip3 install --break-system-packages -r requirements.txt
       - name: Generate Docs
         run: make SPHINXOPTS="-W --keep-going"