test/linux_runner_spec_spec: use unversioned container

[cho-m]

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This test doesn't verify anything related to Homebrew's CI versions so can use the unversioned container to minimize migration diff.

[Copilot]

Pull request overview

Updates the LinuxRunnerSpec unit test fixture to use an unversioned Homebrew container image, reducing churn when CI base images (e.g., Ubuntu versions) change elsewhere in the codebase.

Changes:

• Switch the test’s container.image from ghcr.io/homebrew/ubuntu22.04:main to ghcr.io/homebrew/brew:main.

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[MikeMcQuaid]

Makes sense!