Support "TZDIR" environment variable

Summary

Orage crashes without changing the time zone if "/usr/share/zoneinfo" does not exist.

Description

Currently, in Orage, the directory where "zone.tab" exists is fixed to "/usr/share/zoneinfo".

In the Guix System I use, "/usr/share/zoneinfo" does not exist, so Orage crashes without changing the time zone.

Environment

• Guix System (Linux 6.1.64 #1 SMP PREEMPT_DYNAMIC 1 x86_64 GNU/Linux)
• Orage 4.18.0

Steps to reproduce

1. Run Orage
2. Create a new schedule (or edit an existing schedule)
3. Click the time zone button (e.g. Asia/Tokyo)

Expected behavior

A new frame window will appear with the time zones listed.

Actual behavior

The new frame window is not displayed, and Orage crashes without changing the time zone.

(orage:29796): orage-WARNING **: 00:55:15.942: file error (/usr/share/zoneinfo): No such file or directory

(orage:29796): orage-WARNING **: 00:55:15.942: zone.tab file open failed (zoneinfo/zone.tab): No such file or directory

(orage:29796): orage-CRITICAL **: 00:55:15.942: nftw error in file handling: No such file or directory