Update SF build to run E2E tests every time

The E2E tests will be run on a dedicated TeamCity build agent, currently
running Windows XP though it will be upgraded to Windows 7 very soon. It
is in a separate agent pool on TeamCity, so no builds will normally be
sent to that agent -- which means that it will never have a build
running at the same time as a set of Selenium tests (which could slow
down the build *and* the tests considerably).