generate-cask-ci-matrix: fix Linux runner for single-arch casks

[p-linnane]

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I used Claude Code, manually reviewed the code, validated locally.

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Casks that only support one macOS architecture (arm64-only) but both Linux architectures were missing the second Linux runner from the CI matrix. The runner product only used macOS architectures, an Intel Linux runner paired with arm64 was skipped as non-native.

Fix by including Linux architectures in the product and guarding against macOS runners simulating architectures that don't exist on macOS.

[Copilot]

Pull request overview

Fixes CI matrix generation for casks that are single-arch on macOS (e.g., arm64-only) but support both Linux architectures, ensuring the missing Linux runner is included.

Changes:

• Expand the runner/architecture product to include Linux architectures for Linux-supported casks.
• Add a guard to prevent macOS runners from generating simulated-arch jobs that aren’t valid for macOS in the current cask context.
• Add a new spec fixture/assertion covering a macOS-arm64-only cask that supports Linux.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/dev-cmd/generate-cask-ci-matrix.rb	Includes Linux architectures in job generation and skips invalid macOS simulated-arch combinations.
Library/Homebrew/test/dev-cmd/generate-cask-ci-matrix_spec.rb	Adds a new cask scenario and expectation for runner filtering behavior.

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[Copilot]

Pull request overview

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

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[MikeMcQuaid]

Thanks!