backends/lxc/create-testbed: use fewer different sources.lists

When creating a lxc testbed, we change the apt sources.list multiple times. Initially, the lxc-debian template creates one via lxc-create. This has been called from autopkgtest-build-lxc, which has its own ideas about sources.list and replaces it. That happens to be called from backends/lxc/create-testbed, which again has its own ideas and replaces the sources.list again. If any of these sources.lists does not work, the setup fails.

In order to simplify this, we remove the one from autopkgtest-build-lxc from the equation. Rather than having backends/lxc/create-testbed replace the sources.list, we tell autopkgtest-build-lxc to just use the one we'd be overwriting directly.

Link: https://lists.debian.org/debian-ci/2023/05/msg00000.html