Refactor: store current_example in a local.

* There's no need to call `RSpec.current_example`
  many times in this method.
* This wasn't working correctly for specs that
  create an example group and run it (from within
  the spec itself), as that caused `RSpec.current_example`
  to get cleared when the locally created example
  group completed, even though the spec that created
  the example group had not yet completed.  For an
  example of this, see this rspec-its PR:

https://github.com/rspec/rspec-its/pull/1#discussion_r6677739