autopkgtest: mark networkd-test.py as flaky

See https://github.com/systemd/systemd/issues/18357
and https://github.com/systemd/systemd/issues/18196

debian/tests/control

@@ -52,7 +52,7 @@ Depends: systemd,
   systemd-sysv,
   policykit-1,
   dnsmasq-base
-Restrictions: needs-root, isolation-container
+Restrictions: needs-root, isolation-container, flaky
 
 Tests: build-login
 Depends: systemd,