formula_installer: add fallback to use tap formula

[nandahkrishna]

The keg formula isn't present in some old bottles. Use the tap
formula as a fallback when this is the case.

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A few formulae in linuxbrew-core were bottled over 3-4 years ago, and their bottles do not seem to contain the .brew directory within the prefix. This PR adds a fallback to use the tap formula in such cases (rescuing from FormulaUnavailableError seems to do the trick). These formulae will be rebottled after the core merging, but it may still be a problem for external taps with old bottles.

CC @iMichka

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, thanks @nandahkrishna!