cmd/cache: print cask cache paths for Linux platforms

[krehel]

• brew --cache skipped every Linux combination for casks, so --os=linux printed nothing and --os=all never showed Linux paths even for casks with an on_linux block or an AppImage.
• Evaluate each OS and arch with refresh_for_tag and decide with platform_supported?, which already covers depends_on macos:, depends_on arch: and blank url/sha256, so --cache and fetch agree on what a cask supports.
• Warn with the same wording as brew fetch for Formulas
• Make Cask#platform_supported? public

Reproduce on main:

brew --cache --cask --os=linux actual => prints nothing, exit 0

With this change it prints the Actual-linux-arm64.AppImage cache path.

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Claude Code (Fable 5.1) drafted the change and the spec. I reviewed the diff, ran brew lgtm on macOS and exercised the command on a Fedora 44 container against HEAD.

[Copilot]

🟡 Changes recommended

API-backed casks retain the initially merged variation, and generic --os=macos bypasses macOS support checks.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds Linux cask cache-path support while aligning platform validation with fetch behaviour.

Changes:

• Evaluates casks for each requested OS and architecture.
• Warns for unsupported platform combinations.
• Makes Cask#platform_supported? public and expands tests.

File summaries

File	Description
Library/Homebrew/cmd/--cache.rb	Resolves and validates cask cache paths by platform.
Library/Homebrew/cask/cask.rb	Exposes platform support validation publicly.
Library/Homebrew/test/cmd/--cache_spec.rb	Tests Linux paths and unsupported platforms.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Balanced

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[Copilot]

🟡 Changes recommended

The new support check suppresses valid cache paths for source-loaded casks without installable artifacts.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

[MikeMcQuaid]

Thanks! One comment then looks good.