Temporary fix for Win EWS bots.
Don't build DRT since it has already been built in the build step.

* Scripts/webkitpy/tool/steps/runtests.py:
(RunTests.run):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142180 268f45cc-cd09-0410-ab3c-d52691b4dbfc