Make sure koji_build job runs don't get stuck in pending state

[nforro]

koji_build job runs can get stuck in pending state, for example when there is an infrastructure error and Koji doesn't even start the build. This breaks assumptions for the check that is supposed to prevent submitting a build for the same NVR more than once, and makes retriggering impossible.

Unfortunately the solution won't be as simple as the one for bodhi_update jobs because the handler doesn't block and wait for a Koji task to finish, so some sort of a babysitting task will most likely be required.