Set ENV[HOMEBREW_BREW_FILE]

Thus install.rb gets the path to the instantiating brew process rather than whichever is first in the PATH. Not to mention Homebrew doesn't *have* to be in the PATH.

Library/Homebrew/global.rb

@@ -38,6 +38,10 @@ else
   HOMEBREW_CACHE=Pathname.new("~/Library/Caches/Homebrew").expand_path
 end
 
+if not defined? HOMEBREW_BREW_FILE
+  HOMEBREW_BREW_FILE = ENV['HOMEBREW_BREW_FILE'] or `which brew`.chomp
+end
+
 HOMEBREW_PREFIX = Pathname.new(HOMEBREW_BREW_FILE).dirname.parent # Where we link under
 HOMEBREW_REPOSITORY = Pathname.new(HOMEBREW_BREW_FILE).realpath.dirname.parent # Where .git is found
 

Library/Homebrew/install.rb

 #!/usr/bin/ruby
-HOMEBREW_BREW_FILE = `which brew`.strip
-
 require 'global'
 
 def text_for_keg_only_formula f

bin/brew

@@ -6,6 +6,7 @@
 Dir.getwd rescue abort "The current working directory doesn't exist, cannot proceed."
 
 HOMEBREW_BREW_FILE = File.expand_path __FILE__
+ENV['HOMEBREW_BREW_FILE'] = HOMEBREW_BREW_FILE
 
 def homebrew_rubylib_path
   lib_path = "/../../Library/Homebrew"