Closes#596
`homebrew.brewPrefix` defaulted to the bin directory (`/opt/homebrew/bin`),
not the actual Homebrew prefix (`/opt/homebrew`). This misled users into
writing `${config.homebrew.brewPrefix}/bin`, producing the broken path
`/opt/homebrew/bin/bin`.
Replace it with `homebrew.prefix`, which has correct semantics matching
`brew --prefix`. The old `brewPrefix` option is removed using
`mkRemovedOptionModule`, which catches both users who set the option and
users who read it in custom code. A warning also fires if the new `prefix`
value ends with `/bin`, catching users who copy the old value verbatim.
8c29e146dd