chromium/update: Update Debian binaries as well.

This cases the Debian binaries to be fetched from Google's official APT
repository. If we aren't able to find a package from the APT repository,
it's very likely that it already got deleted upstream and we need to
fallback to mirrors instead.

Unfortunately, we can't use mirrors for updating, because Google doesn't
sign the Debian packages themselves and only the release files.

We're going to hook it into a Chromium updater soon, making the sha256
hashes publicly available, so if it is missing, we can still put the
sha256 manually into sources.nix, without risking anything by blindly
fetching from one of the provided mirrors.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>