Helping out with -DCLOBBER_CACHE_ALWAYS

Tom Lane complained the other day that we needed some more buildfarm animals building with the -DCLOBBER_CACHE_ALWAYS setting, so I set one up yesterday. It's the same machine as nightjar, but this runs at a different time of the day. It's easy enough to do, but the tests take a very long time to run. Compare the times for the new machine friarbird and nightjar, which does substantially more work in about 4% of the time. In fact, the ratio of time would be even higher but for the presence of some constant factors (e.g. build time). So I set this to run around midnight my time every day, and only for the main development branch. That way it should not stress either the VM it runs on or the host machine.