Switch from cdn.debian.net to http.debian.net

Let's see whether it works nowadays, we hit:

| W: Failed to fetch bzip2:/var/lib/apt/lists/partial/cdn.debian.net_debian_dists_sid_main_source_Sources  Hash Sum mismatch
| W: Failed to fetch bzip2:/var/lib/apt/lists/partial/cdn.debian.net_debian_dists_sid_main_binary-amd64_Packages  Hash Sum mismatch
| W: Failed to fetch bzip2:/var/lib/apt/lists/partial/cdn.debian.net_debian_dists_sid_main_i18n_Translation-en  Hash Sum mismatch

in today's automated builds, so it's worth a try...