--prefix: use opt_prefix when available.

Fixes #1952.

Library/Homebrew/cmd/--prefix.rb

@@ -11,7 +11,9 @@ module Homebrew
     if ARGV.named.empty?
       puts HOMEBREW_PREFIX
     else
-      puts ARGV.resolved_formulae.map(&:installed_prefix)
+      puts ARGV.resolved_formulae.map { |f|
+        f.opt_prefix.exist? ? f.opt_prefix : f.installed_prefix
+      }
     end
   end
 end