makesourcepackageupload task fails if dpkg-source is missing

The makesourcepackageupload task fails if the given environment lacks the dpkg-dev package and therefore the dpkg-source utility.

In #1015 (closed), we agreed that default environments should use debootstraps default variant. As dpkg-dev is not priority important, it no longer is part of the default environment. Hence makesourcepackageupload fails.