[Drag & Drop blocks] - Dropping insertion point is shown over the toolbar

## Description

While testing the implementation of the dropping insertion point, a [bug was found](https://github.com/WordPress/gutenberg/pull/39891#pullrequestreview-927855362) where the indicator shows up on top of the toolbar.

**Screenshots**

https://user-images.githubusercontent.com/14905380/161098996-49e32833-ec2a-4cbf-8642-f76b45b500d6.mov