cask/audit: always enable codesign audit for casks

[bevanjkay]

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Enable audit_signing all the time, as discussed in Homebrew/homebrew-cask#170345 (comment)
Previously this was only applied to "new" casks or when the --strict flag was passed.

This change would give us a better indication of how widespread unsigned binaries are in our official cask repositories.

[MikeMcQuaid]

Good call, shipping because it seems we have sufficient consensus to at least try this. Thanks @bevanjkay!