Don't copy the ENV variable from the users's external environment into
construction environments.  SCons uses this as a dictionary of environment
values for the commands it executes.

BUG=none
TEST=build with the variable ENV set in the user environment

Review URL: http://codereview.chromium.org/173294

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2750 ce2b1a6d-e550-0410-aec6-3dcde31c8c00