formula_installer: skip attestations on local_bottle_path

[woodruffw]

This should fix observed failures on Homebrew/homebrew-core#177256 and Homebrew/homebrew-core#177260.

TL;DR: Formula#bottle is not present when a bottle is being installed from a bottle path, e.g. on CI. This is not a normal user installation path and there's no attestation to verify in these cases (since the bottle is still being produced), so we just skip it outright.

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[MikeMcQuaid]

Looks good thanks @woodruffw!