Replace repology datasource with custom Homebrew datasource

[nikobockerman]

repology.org aggressively rate-limits Renovate, so the gcc/llvm major
version lookups fail (renovatebot/renovate#38036). Replace the repology
datasource with a customDatasources entry that queries the Homebrew
formulae API (formulae.brew.sh) directly, which tracks exactly what
Homebrew ships and additionally provides homepage/source links for
Renovate PR bodies.

[Copilot]

Pull request overview

This PR updates the repository’s Renovate configuration to avoid Repology rate-limiting by switching GCC/LLVM version lookups to a custom Renovate datasource backed by the Homebrew Formulae API (formulae.brew.sh).

Changes:

• Added a customDatasources.homebrew datasource in Renovate config pointing at the Homebrew formula JSON API and transforming responses into Renovate’s releases format (plus homepage/source metadata).
• Updated the Homebrew toolchain version annotations in the homebrew-packages-setup composite action to use datasource=custom.homebrew instead of datasource=repology.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/renovate.json5	Adds a custom Renovate datasource for Homebrew formula version/homepage/source lookups.
.github/actions/homebrew-packages-setup/action.yaml	Switches GCC/LLVM Renovate annotations to use the new custom Homebrew datasource.

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