dev-cmd/bottle: check for prefix when not /usr/local

[cho-m]

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Fixes incorrectly marking bottles as relocatable, e.g.

https://github.com/Homebrew/homebrew-core/blob/425d4ea43d36e7d03ce894268dd42aef5236bb54/Formula/p/pkgconf.rb#L34-L36

This cannot be done for /usr/local as it is used outside Homebrew. Other default prefixes are Homebrew-specific.