audit: resolve untapped core dependencies

[MikeMcQuaid]

• Restore API access for external formula audits when a parent command
  automatically disabled it.
• Preserve an explicitly disabled API setting.

Fixes #23539

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OpenAI Codex 5.6 Sol xhigh with local review and testing.

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[Copilot]

Pull request overview

This PR adjusts brew audit to temporarily re-enable API access when auditing external (non-core) formulae in cases where API access was automatically disabled by a parent command (e.g. when Homebrew/core is untapped), while leaving explicitly disabled API settings untouched.

Changes:

• Re-enables API access during external formula audits when HOMEBREW_NO_INSTALL_FROM_API was automatically set.
• Adds an RSpec example covering the automatically-disabled API scenario for external formula audits.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/dev-cmd/audit.rb	Clears automatically-set “no API” env vars around external formula auditing so dependencies can be resolved via the API.
Library/Homebrew/test/dev-cmd/audit_spec.rb	Adds a regression test to ensure API access is re-enabled for external formula audits when it was automatically disabled.

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