LF build: use dedicated Selenium build agent only

Running the E2E tests on a Windows build agent has been unsuccessful so
far: Selenium starts but immediately stops. Until/unless the problem can
be figured out, switch back to running the E2E tests from the Linux
build agent, and using a dedicated Selenium-only build agent to serve up
the Chrome browser. (And/or Firefox, IE, Opera if we add those to our
testing environment at a later date.)