cask: add audit for incorrect signing

[SMillerDev]

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Since often upstreams suggest people disable quarantine instead of fixing their broken signatures, we should probably check if the signature is valid in the audit.

[BrewTestBot]

Review period will end on 2022-08-02 at 12:34:17 UTC.

[BrewTestBot]

Review period ended.

[SMillerDev]

Changed it to a separate flag.

[SMillerDev]

Only the actual signing command is left out for now since that depends on a specific macOS version for output checking. Otherwise I think it is good to go.

[bevanjkay]

@SMillerDev Looks like this commit is causing a couple of false positive errors - https://github.com/Homebrew/homebrew-cask/runs/7859932752?check_suite_focus=true https://github.com/Homebrew/homebrew-cask-versions/runs/7859526329?check_suite_focus=true

I don't have time to fully check it out (it's late here), but just flagging this PR as I've traced the errors back to here.

[SMillerDev]

Okay, somehow my artifact filtering isn't working. I would have expected this line to output a list of only eligible artifacts.

artifacts = cask.artifacts.select { |k| k.is_a?(Artifact::Pkg) || k.is_a?(Artifact::App) }