test: switch to object spread in common/benchmark.js

PR-URL: #30309
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common/benchmark.js

@@ -18,7 +18,7 @@ function runBenchmark(name, args, env) {
 
   argv.push(name);
 
-  const mergedEnv = Object.assign({}, process.env, env);
+  const mergedEnv = { ...process.env, ...env };
 
   const child = fork(runjs, argv, {
     env: mergedEnv,