cbuildbot_launch always does an internal checkout before launching cbuildbot, but if the build config to be launched is external, this is wasteful (though cbuildbot should do the right thing).

It's not possible for cbuildbot_launch to examine build configs, and so it can't tell if the build is internal or external from the config name.

However, the chromite recipe does know, and we might be able to take advantage of this somehow.

PS: This does not prevent correct builds, just wastes time/bandwidth/disk io at the beginning of each build.