add JDK 12 community build

JDK 12 "overall feature set is frozen" (according to https://openjdk.java.net/projects/jdk/12/) as of Feb 13 2019)

we might as well install an early-access JDK 12 build and try it, see what happens, perhaps it will turn up something we ought to do something about

ea builds are here: https://jdk.java.net/12/

behemoth 1 has the lightest community build load at present