image builds set trixie as the codename for unstable images

mmdebstrap and simpleimagebuild extract the codename from /etc/os-release.

This works fine in most situations, but gets it wrong for unstable, where the codename will be that of the next Debian Stable release.

e.g. https://debusine.debian.net/work-request/160/

Either we should stop pulling it from /etc/os-release or we should specifically ignore that value for unstable image builds.

This will matter once we start putting images into a debian:environments collection.