system_time_zone is incorrect on Windows when TZ is set

On Windows, the global variable system_time_zone doesn't properly reflect the used time zone when the environment variable TZ is set.

For example, on my system configured for region Germany, but TZ is set as "CST-8":
SELECT @@time_zone, @@system_time_zone, TIMEDIFF(NOW(), UTC_TIMESTAMP());
SYSTEM Europe/Berlin 08:00:00
Whereas in Linux it returns
SYSTEM CST 08:00:00