Re: [dev-servo] Trychooser support now live!

The `build` chooser has been renamed `linux`, and we now have a `mac` and `windows` chooser. Each one of these uses a builder that builds and runs unit tests (but not heavy things like WPT). I didn't think "just test a mac build" would be something we'd need, but I ended up needing that yesterday

[dev-servo] Trychooser support now live!

Trychooser support is now live on homu! This means that instead of `@bors-servo try`, you can run `@bors-servo try=wpt` to just run linux-rel-wpt and linux-rel-css. This is preferable to a full try; which runs all the builders and is wasteful. It