cask/audit: require "macho" before use

[chenrui333]

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Fixes uninitialized constant Cask::Audit::MachO raised during online cask audits.

What

Cask::Audit#min_os_from_bundle calls MachO.open (added in #20119) but the macho gem was never required in this scope, so the constant is undefined when the min-OS binary check runs. This loads the gem lazily inline, matching the existing pattern in os/mac/mach.rb (which requires it at point of use to keep it off the Linux load path).

Why

Any brew audit --cask --online that reaches the binary min-OS check crashes instead of auditing. This is currently breaking automated cask bump CI.

Reproduce

Surfaced by Homebrew/homebrew-cask#277654 (failed CI run):

brew audit --cask --online emacs-app@nightly
...
exception while auditing emacs-app@nightly: uninitialized constant Cask::Audit::MachO

After this change the same command audits without raising.

[Copilot]

Pull request overview

Fixes Cask::Audit online cask audits raising uninitialized constant Cask::Audit::MachO by ensuring the macho gem is required before MachO.open is called in the bundle minimum-OS audit path.

Changes:

• Lazily require "macho" immediately before calling MachO.open in cask_bundle_min_os.
• Prevents online brew audit --cask --online from crashing when this code path is exercised.

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[MikeMcQuaid]

Thanks!