Configuration of buildd_amd64-x86-grnet-01 for buster-security

[Moritz Mühlenhoff <jmm@inutil.org>]

Hi buildd team,
a while ago the security buildds were configured so that a
source only upload with amd64.buildinfo got renamed to IIRC
a +buildd suffix. Don't remember the final details, but it
successfully avoided problems with source uploads for quite
a while now.

It seems however that buildd_amd64-x86-grnet-01 isn't configured
with that setting since the php7.3_7.3.29-1~deb10u1_amd64 upload
by Ondrej exposed the issue again.

Could you have a look and sorry for the vague problem description,
it worked so well, that I've been getting blurry on the scope of
the original problem :-)

Cheers,
        Moritz