Use ubuntu-latest and add constant for arm linux runner

[cho-m]

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For ubuntu-latest:

• Not sure why dispatch bottle is using ubuntu-22.04 when all other workflows use ubuntu-latest so switched it
• Trying out if it works for Cask CI. Can switch this one back if any issues.

For ubuntu-22.04-arm:

• There is no "latest" label so creating a constant to reuse. Particularly want to use cross-repo like at https://github.com/Homebrew/homebrew-core/blob/main/cmd/determine-rebottle-runners.rb#L25
• This doesn't have to be updated at same time as migration (as we run in container) but we do need to update it every 2 years as GitHub has historically removed images outside standard support so I expect a similar situation for partner images.

[Copilot]

Pull request overview

This PR aligns Linux GitHub Actions runner labels across Homebrew developer commands and runner-matrix generation by switching x86_64 Linux to ubuntu-latest and centralizing the ARM Linux runner label behind a reusable constant.

Changes:

• Switch Linux x86_64 runner label from ubuntu-22.04 to ubuntu-latest in cask CI matrix generation and bottle-dispatch logic.
• Introduce OS::LINUX_CI_ARM_RUNNER (ubuntu-22.04-arm) and use it wherever the ARM Linux runner label is needed.
• Update affected specs to match the new runner labels/constant usage.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Library/Homebrew/test/dev-cmd/generate-cask-ci-matrix_spec.rb	Updates expectations to use ubuntu-latest for x86_64 and the new ARM runner constant.
Library/Homebrew/test/dev-cmd/determine-test-runners_spec.rb	Uses the new ARM runner constant in test runner lists/env setup.
Library/Homebrew/os.rb	Adds OS::LINUX_CI_ARM_RUNNER constant for reuse.
Library/Homebrew/github_runner_matrix.rb	Replaces hardcoded ARM runner label with OS::LINUX_CI_ARM_RUNNER.
Library/Homebrew/dev-cmd/generate-cask-ci-matrix.rb	Switches x86_64 Linux to ubuntu-latest and uses OS::LINUX_CI_ARM_RUNNER for ARM.
Library/Homebrew/dev-cmd/dispatch-build-bottle.rb	Switches Linux x86_64 to ubuntu-latest and uses OS::LINUX_CI_ARM_RUNNER for ARM dispatch.

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[MikeMcQuaid]

Makes sense, thanks!