correctly obtain failed koji build logs

[TomasTomecek]

if a koji build fails, sadly this doesn't do anything:

koji_logs = self.client.getBuildLogs(self.build_or_task_id)

koji api docs:

This method will only return logs for builds that are complete. If a build is in progress, failed, or canceled, you must look at the build's task logs instead (see listTaskOutput).

Instead, we need to manually traverse the task hierarchy and select the right buildArch task, which this commit implements.