Don't use SUSE mirror redirect service for Linaro repo

The service used to host the Linaro custom APT repository, download.opensuse.org, is a mirror service which automatically chooses a mirror local to the user. Not all of the SUSE mirrors contain the packages built by build.opensuse.org, so sometimes we get redirected to a mirror which doesn't have our packages and thus causes a build failure. This seems to happen in most cases.

Switch to a concrete mirror (hosted by gwdg.de) to workaround the mirror service.

Seems like one may use any mirror from 0 under the column Build Service.