Time zones of Earth are changed several times per year, both for future and past dates. In order to ensure that our clocks display the correct time, Guix needs to keep our time zone package (tzdata) up to date. However, the current situation makes this impractical, because changing tzdata causes ~1400 package rebuilds per-architecture (`guix refresh -l tzdata`). We should make sure that packages which use time zones look up the time zones dynamically, at run-time, rather than recording a store reference to the tzdata package, which will be obsolete in months, at the latest. I used `guix graph --type=reverse-package tzdata` to start, and found that the main culprit here is bluez, which depends on tzdata via libical. Using the attached patch, we can make libical look up time zones at runtime with the $TZDIR environment variable. Bluez still builds with this; I'm not sure what it does with libical and if it still works correctly. However, this makes evolution-data-server test suite fail. I'm not sure how to fix evolution-data-server correctly. The fine points of looking up time zones in evolution-data-server were already discussed: