[configuration] Don't suggest/allow starting another prebuild when one is running

[svenefftinge]

In configuration I can start a prebuild. When that is running tze "Run Prebuild" button is still green and active.
When I click it I loose the current logs and see an error message, telling me that I cannot start a prebuild because one is running already.

[gtsiolis]

The initial specs (#4948 (comment)) included a Cancel Prebuild when a prebuild was pending or running.
Does this sounds better? Cc @jankeromnes @svenefftinge

Pending or Running Prebuild

[atduarte]

Should the button in the Project > Prebuilds page have the same behavior? Related with #5374

[jankeromnes]

Okay, I tried implementing a Cancel Prebuild button in #5474, but ended up backpedalling because this would require a new server method (e.g. server.cancelPrebuild, next to server.triggerPrebuild) in order to properly set the Prebuild state as CANCELLED (otherwise, if you just call server.stopWorkspace(prebuildWorkspaceId) as I did, the prebuild will get "cancelled" but then show up as SUCCESSFUL in the UI 🤦).

That PR is already too big, so disabling the Run Prebuild button for now (only disabled when a prebuild is running) and leaving the Cancel Prebuild button/feature as a follow-up.