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.