bazelbot: don't run bot if already running

[codyoss]

Sometimes today we get into cases where the processing time exceeds the schedule we run bazelbot at. When this happens we can run multiple bazelbots and get into cases that there is contention between certain commits and it causes some of the runs to fail. We should limit the executions of some of these long builds so that only one instance is ran at a time. Maybe a distributed lock, maybe a feature of one of the services we are using, I am not quite sure the best approach to take here.