Fix "run_check_subsystem_order.sh" to use $PYTHON.

Previously it just used /usr/bin/env/python, which would fail if we
only had a "python3" binary.

Fixes bug 40095; bugfix on 0.4.4.1-alpha.

changes/bug40095

+  o Minor bugfixes (testing):
+    - When running the subsystem order check, use the python binary
+      configured with the PYTHON environment variable. Fixes bug 40095;
+      bugfix on 0.4.4.1-alpha.

scripts/maint/run_check_subsystem_order.sh

@@ -12,6 +12,7 @@ if ! test -x "${INCLUDES_PY}" ; then
 fi
 
 "${TOR}" --dbg-dump-subsystem-list | \
+    "${PYTHON:-python}" \
     "${INCLUDES_PY}" --check-subsystem-order - "${abs_top_srcdir}/src"
 
 echo ok