workflows/docs: remove cache-homebrew-prefix usage in formulae.brew.sh

[ZhongRuoyu]

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This unblocks formulae.brew.sh CI. For reasoning, see Homebrew/formulae.brew.sh#2155.

[Copilot]

Pull request overview

Updates the documentation GitHub Actions workflow to avoid using the cache-homebrew-prefix action when the workflow runs in the Homebrew/formulae.brew.sh repository, unblocking that repository’s CI while keeping the existing caching behavior for other repos.

Changes:

• Gate Homebrew/actions/cache-homebrew-prefix@main behind a github.repository != 'Homebrew/formulae.brew.sh' condition.
• Add a formulae.brew.sh-specific step that installs vale and ruby via brew install instead.

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