dashboard: make it possible to force kernel rebuild for an instance

[ramosian-glider]

Right now syzbot does not immediately rebuild the kernel when new patches arrive, instead doing that every several hours. In most cases this is a reasonable tradeoff between having the freshest kernel and spending too many resources on compilation.

But sometimes a kernel commit can be critical to syzbot functioning, and running the old kernel further does not make much sense.

For such cases it would be nice to be able to request a kernel rebuild for a given instance at the earliest convenience.

[dvyukov]

The syz-ci part of this is #1770. Support for immediate rebuilding needs to be done in syz-ci first (it's what builds kernels). And it may be enough for rare rebuild requests.

[ramosian-glider]

WDYT about providing something like https://syzkaller.appspot.com/admin?nudge=ci-instance-name that would set some flag indicating a rebuild request that can be checked in Manager.loop()?