global: set HOMEBREW_PATH if unset.

Author: MikeMcQuaid

Library/Homebrew/global.rb

@@ -53,6 +53,7 @@ def raise_deprecation_exceptions?
 
 require "compat" unless ARGV.include?("--no-compat") || ENV["HOMEBREW_NO_COMPAT"]
 
+ENV["HOMEBREW_PATH"] ||= ENV["PATH"]
 ORIGINAL_PATHS = ENV["HOMEBREW_PATH"].split(File::PATH_SEPARATOR).map do |p|
   begin
     Pathname.new(p).expand_path