CI Linux Incremental: Fix "toctree contains reference to nonexisting document"

[mkoeppe]

Seen in https://github.com/sagemath/sage/actions/runs/8626379356/job/23644513913?pr=37570#step:11:2933

#24 4063.0   [sagemath_doc_html-none]   [spkg-install] [structure] updating environment: 0 added, 1 changed, 31 removed
#24 4063.0   [sagemath_doc_html-none]   [spkg-install] [valuation] /sage/src/doc/en/reference/valuations/index.rst:186: WARNING: toctree contains reference to nonexisting document 'sage/rings/valuation/value_group'
#24 4063.0   [sagemath_doc_html-none]   [spkg-install] [valuation] /sage/src/doc/en/reference/valuations/index.rst:186: WARNING: toctree contains reference to nonexisting document 'sage/rings/valuation/valuation'

(A second round of building sagemath_doc_html in the same run then completes without error.)

Another variant seen in https://github.com/sagemath/sage/actions/runs/8626379356/job/23644514413?pr=37570#step:11:4173:

#24 7383.6   [sagemath_doc_html-none]   [spkg-install]     env = pickle.load(f)
#24 7383.6   [sagemath_doc_html-none]   [spkg-install]           ^^^^^^^^^^^^^^
#24 7383.6   [sagemath_doc_html-none]   [spkg-install] AttributeError: Can't get attribute '_stable_repr_object' on <module 'sphinx.builders.html' from '/sage/local/var/lib/sage/venv-python3.11/lib/python3.11/site-packages/sphinx/builders/html/__init__.py'>
#24 7383.6   [sagemath_doc_html-none]   [spkg-install]