reproducible Debian: add sources.list for rb-mail1 still running buster due to...

reproducible Debian: add sources.list for rb-mail1 still running buster due to mailman2 using python2

Signed-off-by: Holger Levsen <holger@layer-acht.org>

hosts/rb-mail1/etc/apt/sources.list

+deb http://deb.debian.org/debian buster main
+#deb-src http://ftp.de.debian.org/debian buster main
+
+deb http://security.debian.org/debian-security buster/updates main
+#deb-src http://security.debian.org/debian-security buster/updates main
+
+deb http://deb.debian.org/debian buster-updates main
+#deb-src http://ftp.de.debian.org/debian buster-updates main
+