zshrc: find Homebrew's completions on the prefix this machine uses

fpath carried /usr/local/share/zsh-completions, which is wrong twice over:
Apple Silicon puts Homebrew under /opt/homebrew, and formulae install their
completions into share/zsh/site-functions, not the share/zsh-completions of
the formula by that name. The directory existed on neither Mac, so _brew,
_bat, _deno and the rest have been missing all along -- silently, since a
missing fpath entry says nothing.

All four candidates are probed now, HOMEBREW_PREFIX first where a machine
exports it. No `brew --prefix`: that forks on every interactive shell, and
this file already pays for one of those further down.

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