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.