Add TODO for Cargo release cooldown

[MikeMcQuaid]

Add a TODO comment to enable this later.

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OpenAI Codex Sol 5.6 xhigh with local review and testing.

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[Copilot]

Pull request overview

Adds Cargo crates.io release cooldown settings and Cargo resolver configuration to formula sandboxes.

Changes:

• Configures Cargo minimum publish age and unstable resolver settings.
• Adds unit coverage for the sandbox environment variables.

Reviewed changes

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

File	Summary
Library/Homebrew/test/formula_spec.rb	Tests the new Cargo settings.
Library/Homebrew/formula.rb	Configures Cargo cooldown environment variables.

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