use preferred arch if there is more options
https://pagure.io/koji/pull-request/323 introduced option to force an
arch for noarch packages. Nevertheless, it is only task architecture and
not buildroot one. So, if builder can provide more possible archs (32/64
bit), it will pick one of these randomly and can go against preferred arch.
Patch introduces "preferred_arch" option for find_arch and use the
task's one when it works with noarch rpm.
Fixes: https://pagure.io/koji/issue/789