src: fix Environment::GetCurrent() resource leak

Environment::GetCurrent(Isolate*) makes a call to
Isolate::GetCurrentContext(), which returns a Local<Context>. Hence,
every call to GetCurrent(Isolate*) should be made within a HandleScope
or EscapableHandleScope.

Also removed excessive calls to GetCurrent().

/cc @bnoordhuis @indutny 
You're probably aware of this but removing the HandleScope is only safe because `args.Holder()` doesn't create a new `Local<Object>` slot.  I suppose my point is that it's not harmful but if someone later adds code that does create new Locals but forgets to add a HandleScope, then there's a resource leak.  Caveat emptor and all that.