Use Debian Trixie containers

[bedroge]

Trixie containers are now available for RISC-V, which means that we can/should replace the Sid (development) ones by Trixie. It will also allow us to get rid of some patches in the build scripts, e.g. https://github.com/EESSI/filesystem-layer/blob/main/containers/build-or-download-cvmfs-debs.sh#L11 and https://github.com/EESSI/filesystem-layer/blob/main/containers/build-or-download-cvmfs-debs.sh#L68.

There are even some CVMFS packages for Debian 13 already (https://ecsft.cern.ch/dist/cvmfs/cvmfs-2.13.0/), so we can probably restrict the condition in https://github.com/EESSI/filesystem-layer/blob/main/containers/build-or-download-cvmfs-debs.sh#L19 a bit more (only RISC-V?). I'll open a PR when #216 is merged.