Set GUI time zone from geolocation

The wonderful *ipapi.co* service can help identify the time zone of a
device based on the geolocation information for its IP address.  We can
use this to set the `TZ` environment variable in the user session, which
Firefox will respect.  This has the effect of showing the correct time
in e.g. Grafana dashboards, without hard-coding the time zone in the
rootfs image.

overlay/home/user/.xinitrc

@@ -2,6 +2,10 @@
 
 xset s off
 xset -dpms
-systemctl --user import-environment DISPLAY
+
+TZ="$(curl https://ipapi.co/timezone)"
+export TZ
+
+systemctl --user import-environment DISPLAY TZ
 systemctl --user start firefox-marionette.socket firefox
 exec openbox