obs: Get the final build results without races

The OBS API does not give a good way to get the build result for a specific upload: it has endpoints to return the last build log, the current status of the package or the list of the most recent completed build jobs (that is, not jobs that have been interrupted nor jobs that are still building).

This means that printing the build log will always be subject to race conditions, but by using the job history endpoint we can at least ensure that the pipeline succeeds only if the build matching the original upload revision succeeded.