formula_cellar_checks: fix edge case handling in check_binary_arches

[carlocab]

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Currently, if formula foo ships both universal and non-native
binaries and foo is on both allowlists, then brew audit --strict
errors out with an empty error message:

❯ brew audit --strict foo
foo:
  *
Error: 1 problem in 1 formula detected

Let's fix this (admittedly obscure) corner case by returning early when
a formula is present on both allowlists.

[BrewTestBot]

Review period will end on 2021-09-21 at 17:08:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.