cbuild: drop cat /etc/apt/sources.list from debian

This file no longer exists by default on SID, so the cat fails and stops our script. Since this command is executed via vmctl exec, it's not easily expanded to a condition and while more -e works, that isn't available on stretch.

So - since this is a basic container anyway before we make any large modifications, the sources list can usually be verified by running that same container locally. We can safely-enough drop the cat.