fix: use 4th private key (index 3) for tx fuzz like before (#215)

this is another change we added while fixing merge conflicts today

src/transaction_spammer/transaction_spammer.star

@@ -12,7 +12,7 @@ def get_config(prefunded_addresses, el_client_context, tx_spammer_extra_args):
 		cmd = [
 			"spam",
 			"--rpc=http://{0}:{1}".format(el_client_context.ip_addr, el_client_context.rpc_port_num),
-			"--sk={0}".format(prefunded_addresses[0].private_key),
+			"--sk={0}".format(prefunded_addresses[3].private_key),
 			"{0}".format(" ".join(tx_spammer_extra_args))
 		]
 	)