Queue should wait to progress until any active master build is finished

[jamiebuilds]

Right now if you have a build on master which has side effects (releasing/publishing packages) that needs to push back to master, Landkid introduces a race condition where the Landkid build can finish and push to master before the master build has completed.

Ideally people wouldn't be publishing in master (and we should provide a step for publishing as part of Landkid as mentioned previously) but we should handle that gracefully if they do.