Install cached bottles if curl --head fails

[boblail]

Fixes #15302

Prior to #15117, if curl --head failed for any reason, brew install would've installed previously-downloaded bottles.

If that was an unintended side-effect of #15117, then this PR restores the previous behavior.

If it was intended, is there a way to brew install a bottle from the filesystem — i.e. a no-network install provided that the given bottle matches the formula's expectations?

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[MikeMcQuaid]

Thanks for the PR @boblail! A few comments.

[MikeMcQuaid]

Thanks again @boblail, another great PR!

[boblail]

Thanks, @MikeMcQuaid! — and thanks for the quick turn-around, as usual!