The tests under test/distro are currently disabled: they took too long to run,
and in some periods we might want to release once per day or every second day.

Still, they were valuable in the past to catch weirdnesses on non-x86
architectures and odd build failures with older versions of C libraries. We
recently had failures that went unnoticed for a while due to new gcc versions
and distribution-default build flags we didn't apply (-flto on gcc 12).

We could prepare images to include at least gcc and make for architectures that
are typically run with qemu TCG -- that usually takes the majority of the time.

There might be some quick possibility to run some steps in parallel, too.

Once this is done, we should evaluate whether to re-enable them for, at least,
CI runs.