When splitting tzdata into two packages, one for "real" use and one for
test suites, I'd hoped it would allow us to update tzdata immediately
after new versions were released.

But, there are still way more than 300 packages depending on tzdata, so
it should be updated on a staging branch:

$ guix refresh -l -e '(@@ (gnu packages base) tzdata)'      
Building the following 335 packages would ensure 635 dependent packages are rebuilt ...

According to NEWS, if we don't update tzdata soon, some time zones will
first become incorrect in Northern Cyprus in 5 days.

"Northern Cyprus switches from +03 to +02/+03 on 2017-10-29"

Any advice? Should we do a staging build? Just be late (the old status
quo)? Something else?