u_trace: use os_get_option instead of getenv

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Tim Van Patten <timvp@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37740>

src/util/perf/u_trace.py

@@ -428,7 +428,7 @@ ${trace_toggle_name}_variable_once(void)
      ;
 
    ${trace_toggle_name} =
-      parse_enable_string(getenv("${trace_toggle_name.upper()}"),
+      parse_enable_string(os_get_option("${trace_toggle_name.upper()}"),
                           default_value,
                           config_control);
 }