cask/audit: fix install of container deps

[bevanjkay]

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After changes to utilise the download queue, the installation of "container dependencies" during brew audit were no longer working as expected. The Homebrew::Install.fetch_formulae method expects a formula installer to be passed to it, but a Formula was being passed instead.

brew audit sabaki --online --signing --cask

audit for sabaki: failed
 - exception while auditing sabaki: undefined method 'formula' for an instance of Formulary::FormulaNamespace3d483f559c610b6e653ea661e1a9bab902f1b0a7dc894c44d53ffb7d93a039a1::P7zip

[MikeMcQuaid]

Thanks!