Move require into method to avoid circular require

There is a nasty circular dependency here:

formula.rb
  requirement.rb
    extend/ENV.rb
      extend/ENV/shared.rb
        formula.rb

Probably the information that the build environment needs from formula
should be passed in at runtime, but that seems hard and I really just
want to be able to run the tests with warnings turned on. :/

Library/Homebrew/requirement.rb

@@ -1,7 +1,6 @@
 require 'dependable'
 require 'dependency'
 require 'build_environment'
-require 'extend/ENV'
 
 # A base class for non-formula requirements needed by formulae.
 # A "fatal" requirement is one that will fail the build if it is not present.
@@ -127,6 +126,7 @@ class Requirement
       if instance_variable_defined?(:@satisfied)
         @satisfied
       elsif @options[:build_env]
+        require "extend/ENV"
         ENV.with_build_environment { yield @proc }
       else
         yield @proc