hellonet: set password for root

otherwise incoming ssh gets a bit fraught

examples/hellonet.nix

@@ -19,7 +19,9 @@ in rec {
   services.sshd = svc.ssh.build { };
 
   users.root = {
-    passwd = "";
+    # the password is "secret". Use mkpasswd -m sha512crypt to
+    # create this hashed password string
+    passwd = "$6$y7WZ5hM6l5nriLmo$5AJlmzQZ6WA.7uBC7S8L4o19ESR28Dg25v64/vDvvCN01Ms9QoHeGByj8lGlJ4/b.dbwR9Hq2KXurSnLigt1W1";
   };
 
   services.dns =