[nexus] fix intermittent failure in test 5.5.5 (#12523)

This commit fixes an intermittent failure in Nexus test 5.5.5. The
failure occurred when Router_1 performed a late upgrade from child to
router after re-attaching to the DUT. This caused a mismatch between
the RLOC16 in the captured ping packets and the RLOC16 saved in the
test info JSON at the end of the test.

Changes:
- Disabled router eligibility for Router_1 in test_5_5_5.cpp after it
  becomes isolated. This ensures it remains a child of the DUT and
  maintains a consistent RLOC16 for the duration of the test.

tests/nexus/test_5_5_5.cpp

@@ -169,6 +169,7 @@ void Test5_5_5(void)
      * - Pass Criteria: N/A
      */
     r[3]->Reset();
+    SuccessOrQuit(r[1]->Get<Mle::Mle>().SetRouterEligible(false));
 
     Log("---------------------------------------------------------------------------------------");
     Log("Step 4: Router_1");