MDEV-29102 system_time_zone is incorrect on Windows when TZ is set

MDEV-19243 introduced a regression on Windows.

In (supposedly rare) case, where environment variable TZ was set,
@@system_time_zone no longer derives from TZ. Instead, it incorrecty
refers to system default time zone, eventhough UTC time conversion
takes TZ into account.

The fix is to restore TZ-aware handling (timezone name derives from
tzname), if TZ is set.

sql/mysqld.cc

@@ -4303,14 +4303,24 @@ static int init_common_variables()
   if (ignore_db_dirs_init())
     exit(1);
 
-#ifdef _WIN32
-  get_win_tzname(system_time_zone, sizeof(system_time_zone));
-#elif defined(HAVE_TZNAME)
   struct tm tm_tmp;
-  localtime_r(&server_start_time,&tm_tmp);
-  const char *tz_name=  tzname[tm_tmp.tm_isdst != 0 ? 1 : 0];
-  strmake_buf(system_time_zone, tz_name);
-#endif /* HAVE_TZNAME */
+  localtime_r(&server_start_time, &tm_tmp);
+
+#ifdef HAVE_TZNAME
+#ifdef _WIN32
+  /*
+   If env.variable TZ is set, derive timezone name from it.
+   Otherwise, use IANA tz name from get_win_tzname.
+  */
+  if (!getenv("TZ"))
+    get_win_tzname(system_time_zone, sizeof(system_time_zone));
+  else
+#endif
+  {
+    const char *tz_name= tzname[tm_tmp.tm_isdst != 0 ? 1 : 0];
+    strmake_buf(system_time_zone, tz_name);
+  }
+#endif
 
   /*
     We set SYSTEM time zone as reasonable default and