This allows you to download build.log for all arch for a specific Koji
build.
I considered whether to download all available logs for the build, or
just build.log. Downloading all logs was more difficult to do and also
required sending more Koji queries. For the moment, the code supports
build.log only, which is the only one we have some use for (rpmgrill).
Maybe log a warning if build_log and action not in (...)? Or is it too defensive?