[github-actions] fix NCP border routing setup (#2622)

The commit fixes issue in the github action for testing NCP border
routing.

Without `sudo` the option cannot be set successfully, and border
routing cannot work in the test. Currently the expect script isn't
strong enough to detect this problem. I'll enhance the script soon.

tests/scripts/ncp_mode

@@ -172,8 +172,8 @@ setup_infraif()
     else
         echo "backbone1 already exists."
     fi
-    sysctl -w net.ipv6.conf.backbone1.accept_ra=2
-    sysctl -w net.ipv6.conf.backbone1.accept_ra_rt_info_max_plen=64
+    sudo sysctl -w net.ipv6.conf.backbone1.accept_ra=2
+    sudo sysctl -w net.ipv6.conf.backbone1.accept_ra_rt_info_max_plen=64
 }
 
 test_setup()