This took **a lot** of time, as autopkgtest-build-qemu has been able to
build non-x86 for less than a release (version 5.17, unreleased),
and it isn't meant to be (easly) used on Ubuntu (and unfortunately GH
Actions only support Ubuntu).

This is extremely fragile; adding a CPU core breaks something, adding a
MB of RAM breaks something, installing recommended packages breaks
something... you get it. But it works. Beautiful, isn't it?

This currently tests amd64, arm64. armhf, i386 and ppc64el. s390x seems
unsupported by autopkgtest-build-qemu.

If you're curious to see how many trials it took, have a look at this
test branch:
github.com/Tachi107/pistache/tree/actions-autopkgtest-multiarch-test