src: use local isolate instead of args.GetIsolate

While stepping though SetupPromises I noticed that the environments
Isolate is used but not when creating the string "_setupPromises".

Is there a reason for using args.GetIsolate() instead of using the
environments isolate? I see that GetIsolate() is an inline call, but
could there be situations where it returns a different Isolate?
If not perhaps using the local isolate variable would be a litte
clearer.

PR-URL: #14768
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node.cc

@@ -1230,7 +1230,7 @@ void SetupPromises(const FunctionCallbackInfo<Value>& args) {
 
   env->process_object()->Delete(
       env->context(),
-      FIXED_ONE_BYTE_STRING(args.GetIsolate(), "_setupPromises")).FromJust();
+      FIXED_ONE_BYTE_STRING(isolate, "_setupPromises")).FromJust();
 }