Fix brew doctor with HOMEBREW_INSTALL_FROM_API

[Rylan12]

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Fixes #12356

I could have sworn I fixed this in #12333 but I must have made some change and forgotten to test it out fully. Either way, this should fix the check_deleted_formulae check in brew doctor so that if HOMEBREW_INSTALL_FROM_API is set, kegs with their tap set to homebrew/core and kegs with a blank tap but which are available via the API won't be listed as being deleted.

This also raised another interesting issue which is that installing with HOMEBREW_INSTALL_FROM_API (when homebrew/core isn't tapped) leaves the tap entry in INSTALL_RECEIPT.json blank. This should probably be changed to be homebrew/core. However, since people have been using it, it's probably too late to not have the blank tap condition here.

I'm way too tired to think this through completely right now, though, so I'll just leave this as-is and maybe re-visit in the morning.

[BrewTestBot]

Review period skipped due to critical label.