lib: use existing isWindows variable

Author: sinkhaha

lib/internal/errors.js

@@ -787,7 +787,7 @@ const fatalExceptionStackEnhancers = {
     // However, fatal error are handled differently and we cannot easily
     // highlight them. On Windows, detecting whether a console supports
     // ANSI escape sequences is not reliable.
-    if (process.platform === 'win32') {
+    if (isWindows) {
       const info = internalBinding('os').getOSInformation();
       const ver = ArrayPrototypeMap(StringPrototypeSplit(info[2], '.'),
                                     Number);