Run on s390x in CI

Recently a bug in git-last-modified(1) was discovered when running tests on s390x.

I've managed to reproduce the issue by cross-compiling and emulating using QEMU: https://www.youtube.com/watch?v=IUxMkr_QUjU

But it might be useful to have CI run on this architecture automatically.

I've found a few resources on the topic;

• https://github.com/simonw/til/blob/main/docker/emulate-s390x-with-qemu.md
• https://docs.gitlab.com/omnibus/development/s390x/

The main question is whether this job will be fast enough? If it's really slow, it might not be worth it. Or maybe only on manual run.