x/build: go builders and trybots for third-party repositories

[nathany]

"It is actually a medium-term goal of mine to support third-party repositories, once I complete some redesign of our build system and redo its scheduler. I'm putting off builder work until after the Go freeze.

"The first trial customer will be grpc-go, which is hosted on Github. There are resource cost and isolation questions, so it'll likely be whitelist-only for a bit, and certain projects will probably only get spare resources which are otherwise idle." - @bradfitz https://groups.google.com/d/msg/golang-dev/SVrNHQU1oEM/b13BeZHVBwAJ

I would like to test one or more packages in the fsnotify organization across all the architectures that Go itself supports.