fix: don't require inspector module on production env (#5228)

because of production env shouldn't use inspector module and such as vercel/pkg custom node runtime remove it.
so we hope egg disable inspector on production env.

closes #5226

lib/egg.js

@@ -3,7 +3,6 @@ const path = require('path');
 const fs = require('fs');
 const ms = require('ms');
 const http = require('http');
-const inspector = require('inspector');
 const EggCore = require('egg-core').EggCore;
 const cluster = require('cluster-client');
 const extend = require('extend2');
@@ -115,7 +114,7 @@ class EggApplication extends EggCore {
         isLeader: this.type === 'agent',
         logger: this.coreLogger,
         // debug mode does not check heartbeat
-        isCheckHeartbeat: inspector.url() === undefined,
+        isCheckHeartbeat: this.config.env === 'prod' ? true : require('inspector').url() === undefined,
       });
       const client = cluster(clientClass, options);
       this._patchClusterClient(client);