brew audit should only run cask audits relying on network calls with --online

[MikeMcQuaid]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

brew audit --skip-style --tap=homebrew/cask should not run checks that require you to be online without --online.

What is the motivation for the feature?

• consistency with formulae behaviour
• make use of, already existing, --online flag
• speed up cask audits
• avoid (currently) broken Homebrew/brew CI: https://github.com/Homebrew/brew/actions/runs/5984512534/job/16235756302?pr=15913#step:7:25

How will the feature be relevant to at least 90% of Homebrew users?

More productive maintainers.

What alternatives to the feature have been considered?

Removing these checks.