Multiple weather instances with different timezones interfere when running in the same process

Issue Description

Multiple weather instances with different timezones interfere when running in the same process:

With two xfce4-weather-plugin instances configured for different timezones (e.g. CEST and EDT), refreshing one instance changes the date/time displayed by the other instance to the same timezone.

Additional Information

I traced this to update_timezone():

g_setenv("TZ", data->timezone, TRUE);

data->timezone is per-instance, but TZ is process-global. Therefore, when multiple weather instances run inside the same process, the last instance that calls update_timezone() changes the timezone subsequently used by all instances through libc time functions.

This is reproducible with a fresh user configuration.

Under X11, each weather instance runs in a separate wrapper-2.0 process, so the problem is hidden by process isolation.

Under xfwl4, multiple weather instances are loaded into the same xfce4-panel process, exposing the problem.

Expected behavior:

Multiple weather-plugin instances should be able to use different timezones independently when running in the same process.

Environment & Versions

• Xfce 4.20 / xfce4-panel 4.21.2-dev-2a21dfa66
• xfwl4 Wayland session
• xfce4-weather-plugin version : 0.12.0-dev-c2497954
• Fresh test user configuration