Export HOMEBREW_PREFIX to build environment

Constructing the path relative to a file in the repository is incorrect
on some supported configurations (i.e., the repository is located in a
different place than the prefix).

Closes Homebrew/homebrew#24418.

Library/ENV/libsuperenv.rb

@@ -25,5 +25,5 @@ class Array
   end
 end
 
-$brewfix = "#{__FILE__}/../../../".cleanpath.freeze
+$brewfix = ENV['HOMEBREW_PREFIX'].freeze
 $sdkroot = ENV['HOMEBREW_SDKROOT'].freeze

Library/Homebrew/extend/ENV/super.rb

@@ -72,6 +72,7 @@ module Superenv
     self['HOMEBREW_CCCFG'] = determine_cccfg
     self['HOMEBREW_OPTIMIZATION_LEVEL'] = 'Os'
     self['HOMEBREW_BREW_FILE'] = HOMEBREW_BREW_FILE
+    self['HOMEBREW_PREFIX'] = HOMEBREW_PREFIX
     self['HOMEBREW_SDKROOT'] = "#{MacOS.sdk_path}" if MacOS::Xcode.without_clt?
     self['HOMEBREW_DEVELOPER_DIR'] = determine_developer_dir # used by our xcrun shim
     self['HOMEBREW_VERBOSE'] = "1" if ARGV.verbose?