benchmark: use cluster.isPrimary instead of cluster.isMaster

`cluster.isMaster` was deprecated. So need to use `cluster.isPrimary`
for benchmark.

Refs: #47981
PR-URL: #48002
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: deokjinkim

benchmark/cluster/echo.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const cluster = require('cluster');
-if (cluster.isMaster) {
+if (cluster.isPrimary) {
   const common = require('../common.js');
   const bench = common.createBenchmark(main, {
     workers: [1],

benchmark/http/cluster.js

@@ -4,7 +4,7 @@ const PORT = common.PORT;
 
 const cluster = require('cluster');
 let bench;
-if (cluster.isMaster) {
+if (cluster.isPrimary) {
   bench = common.createBenchmark(main, {
     // Unicode confuses ab on os x.
     type: ['bytes', 'buffer'],