Investigate possible build failures under QEMU

I'm building Pi images for various customers, sometimes with an almost unchanged image-specs, sometimes with heavy modifications.

In the latter case, I've started seeing failures to build images, due to Python code erroring out (e.g. python3-tz.postinst). Since that rang a bell, I've tried upgrading QEMU packages from bookworm to bookworm-backports, and that did the trick.

In the former case, I haven't seen any failures, but the machine I'm using for that was already using backported QEMU packages, because of other failures (gcc ICEs) started showing up when crossbuilding unrelated packages.

I'm not exactly sure if that's going to be reproducible when building “plain Debian images” (see also #75) at this point but I thought I'd file this issue so that others know about this possible failure. The failures became systematic for the “heavily modified images” after the 12.8 point release, but I have yet to check what happens with vanilla image-specs vs. bookworm QEMU packages.