[EMSUSD-623] Variant selection that affects MayaReference prim does not update Reference

**Describe the bug**
With an Variant Set on a USD prim, where one variant contributes nothing, and the others each contribute a MayaReference prim with different paths, changing the variant selection does not behave as expected after the first time.

**Steps to reproduce**
1. Load the `asset.usda` layer (attached in zip) - make sure `model.usda` and the two rigs are in the same directory
2. Expand the proxy in the outliner to the `/cube` prim -
3. Set the `rig` variant to `layout`.
4. Observe a rig reference loads
5. Set the `rig` variant to `none`
6. Observe the rig reference did not unload.
7. Set the `rig` variant to `anim`
8. Observe the rig reference did not change.

**Expected behavior**
When making a Variant selection that affects the composition of a MayaReference prim, the scene should update accordingly.

**Attachments**

**Specs (if applicable):**
- Centos 7
- Maya 2023.3
- Maya USD 0.23.0
- Pixar USD 21.02

[asset.zip](https://github.com/Autodesk/maya-usd/files/11490979/asset.zip)