perf: two minor perf refactors in InvokeIpcCallback()
1. Allocate the CallbackScope on the stack instead of the heap
2. Skip a redundant call to node::Environment::GetCurrent()
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>