LF build: use new Win7 build agent

There's a new Windows7 build agent configured on the TeamCity server.
Switch the E2E tests to use it rather than the WinXP agent. This will
not be a permanent change, as this build agent will also be available
for normal builds (it is not a dedicated Selenium build agent), but for
now we want to make sure that the Selenium tests will run on Win7.