Use Correct Links For Brew Mirrors

[XXXXRT666]

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Fix #21102

[Copilot]

Pull Request Overview

This PR fixes the URL construction for portable Ruby downloads when using custom brew mirrors via HOMEBREW_BOTTLE_DOMAIN. The change removes the hardcoded /bottles-portable-ruby/ subdirectory path that was incorrectly assumed to exist on all mirror servers.

Key changes:

• Simplified the portable Ruby URL construction for custom bottle domains to use just the filename without assuming internal directory structure

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[MikeMcQuaid]

Thanks!