Handle APT sources in debian.sources as well as sources.list

All pipelines are currently failing in unstable:

$ sed -n '/^deb\s/s//deb-src /p' /etc/apt/sources.list > /etc/apt/sources.list.d/deb-src.list
sed: can't read /etc/apt/sources.list: No such file or directory

The APT sources list in the base image is now a deb822-style file at /etc/apt/sources.list.d/debian.sources instead of the old-style /etc/apt/sources.list.

Change the build-script in salsa-ci.yml to handle whichever of the two files are present.

Change the Dockerfile for the gbp image similarly.