sorbet: Bump more OS-specific tests to higher typed levels

[issyl0]

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code CLI, Claude Opus 5 (High reasoning, Ultracode was too intense 🤯), with human prompting, review and tweaks.

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• Part of my ongoing quest to bump all the test files to at least Sorbet typed: true, because that's a thing we can do.
• In a 🥞 gh stack because I wanted to try them out on a real chain of changes.
• See the commit message body for the full description of the changes.

[Copilot]

Pull request overview

This PR increases Sorbet type coverage in OS-specific RSpec tests by bumping selected spec files from typed: false to typed: true, ensuring these tests typecheck under stricter settings.

Changes:

• Promote Library/Homebrew/test/os/mac/ffi/native_library_spec.rb to typed: true and adjust the test module’s singleton method definition to satisfy Sorbet typing (via T.bind and define_singleton_method).
• Promote Library/Homebrew/test/os/linux/ld_spec.rb to typed: true and refactor the temporary directory setup from a captured local variable to an RSpec let.

Show a summary per file

File	Description
Library/Homebrew/test/os/mac/ffi/native_library_spec.rb	Raises Sorbet strictness for the macOS FFI native library spec and updates method definition/binding for typechecking.
Library/Homebrew/test/os/linux/ld_spec.rb	Raises Sorbet strictness for the Linux ld spec and modernises tmpdir setup using let.

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