Set `time_completed` before calling KojiContentGenerator.
    
    The `KojiContentGenerator.finalize()` needs the `ModuleBuild.time_completed`.
    
    We currently set `time_completed` once the module build transitions
    into `done` state. But we have moved the `KojiContentGenerator` call
    to end of `build` state, so right now it is called before the `time_completed`
    is set. This leads to traceback.
    
    In this commit, the `time_completed` is set before the `KojiContentGenerator`
    call, so it is defined properly.