test_bot/formulae: only run subset of brew tests for portable Ruby

[cho-m]

Currently tests fail on all runners (Tier 1 Linux and Tier 3 macOS). It is difficult to align the setups used between Homebrew/brew and Homebrew/core so can limit to basic sanity of the cask and formula tests.

Also update brew style to run on Tier 1-only so that we don't have to manually update formula list and can keep independent from brew style implementation details.

Portable Ruby PRs have been broken for over a month now. Given we now have a new Ruby update (along with OpenSSL, etc.), this should unblock us:

• portable-ruby 4.0.6 portable-libffi 3.7.1 portable-openssl 4.0.1 homebrew-core#293046

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[Copilot]

Pull request overview

This PR adjusts test_bot’s portable Ruby workflow to avoid failures across unsupported/less-aligned runner environments by (a) restricting when brew style runs and (b) reducing the scope of brew tests executed for portable Ruby.

Changes:

• Run brew style only when Homebrew considers the current environment a “Tier 1 configuration”.
• Restrict brew tests to a subset via --only=cask,formula for portable Ruby runs.

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