Apply prefer-const lint rule

lib/server.js

@@ -52,7 +52,7 @@ function closeServer(timeout, callback) {
     server.removeListener("request", listener);
     server.setTimeout(1);
 
-    let cb = callback;
+    const cb = callback;
     callback = null;
     setTimeout(cb, 1000);
   }, timeout).unref();
@@ -60,7 +60,7 @@ function closeServer(timeout, callback) {
   server.close(() => {
     if (!callback) return;
 
-    let cb = callback;
+    const cb = callback;
     callback = null;
     cb();
   });