Fix to replace current env with a blank one.
Fix to replace current env with a blank one.

Actual builder unitttest environment isn't the same as just running
gmerge_test.py.  This fix captures that

Change-Id: I9e87e32f0025bfb7ec7d31a37c11f6cb2d8dba21

BUG=TreeCloser
TEST=Ran cros_run_unit_test cros-devutils

Review URL: http://codereview.chromium.org/6850002

coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util