LibTimeZone: Default to UTC if parsing the TZ environment variable fails

Commit c3fd455 changed LibTimeZone to fall back to the system time zone
when we fail to parse the TZ environment variable. This behavior differs
from both our LibC and glibc; they abort parsing and default to UTC.

This changes LibTimeZone to behave the same way to avoid a very awkward
situation where some parts of the codebase thinks the timezone is UTC,
and others think the timezone is whatever /etc/timezone indicates.

Userland/Libraries/LibTimeZone/TimeZone.cpp

@@ -99,6 +99,7 @@ StringView current_time_zone()
             return *maybe_time_zone;
 
         dbgln_if(TIME_ZONE_DEBUG, "Could not determine time zone from TZ environment: {}", time_zone);
+        return "UTC"sv;
     }
 
 #ifdef AK_OS_SERENITY