formula_auditor: split out checksum check

[cho-m]

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I think this will allow us to extend audit skip so that we can approve confirmed checksum changes maybe adding a CI-confirmed-checksum-change tag.

Open to different naming.

[MikeMcQuaid]

Looks good! The other case I can think of is the new formula notability checks.

CI-confirmed-checksum-change

Would love something like CI-checksum-confirmed-by-asking-upstream or something super explicit so that people don't just slap it on and ignore the message.