benchmark: coerce PORT to number

Without this fix net/tcp-raw-c2s.js aborts in environments where PORT
is defined. TCPWrap::Connect expects the third arg to be a UInt32.

PR-URL: #23721
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/_http-benchmarkers.js

@@ -8,7 +8,7 @@ const requirementsURL =
   'https://github.com/nodejs/node/blob/master/doc/guides/writing-and-running-benchmarks.md#http-benchmark-requirements';
 
 // The port used by servers and wrk
-exports.PORT = process.env.PORT || 12346;
+exports.PORT = Number(process.env.PORT) || 12346;
 
 class AutocannonBenchmarker {
   constructor() {