ci/docs: disable optional cache pkg

[Borda]

What does this PR do?

resolves crashing docs build on master, preventing the release
https://github.com/Lightning-AI/pytorch-lightning/actions/runs/10265242543/job/28401039067
the dropped step would speed up installation but seems we accumulated to many packages and the cache needs to be cleaned

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📚 Documentation preview 📚: https://pytorch-lightning--20168.org.readthedocs.build/en/20168/

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 fabric: Docs

Check ID	Status
docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to .github/workflows/docs-build.yml.

🟢 pytorch_lightning: Docs

Check ID	Status
docs-make (pytorch, doctest)	success	✅
docs-make (pytorch, html)	success	✅

These checks are required after the changes to .github/workflows/docs-build.yml.

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Thank you for your contribution! 💜

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