[EMSUSD-3027] Pref attribute exported with bad name

**Describe the bug**
We export assets from Maya to USD with the option referenceObjectMode=attributeOnly to get the Pref attribute. It works, but maya-usd sets the attribute name to `pref` and Arnold expects it to be `Pref` to use the standard AOV of the same name.

**Steps to reproduce**
Steps to reproduce the behavior:
1. Create a cube
2. Go to Texturing -> Create Texture Reference Object
3. Select the cube and Export Selection to a USD file with the option referenceObjectMode=attributeOnly (available through scripting only)
4. Load cube in Houdini Solaris ;)
5. Add an Arnold node
6. Check the standard AOV named Pref
7. Render and look at Pref AOV.
8. All is black.

**Expected behavior**
We should see the values of the Pref attribute in the render directly.

**Attachments**
If applicable, add screenshots, sample files, etc to help explain your problem.

**Specs (if applicable):**
- OS & version : Windows 10
- Maya version: Maya 2024.2
- Maya USD version: 0.30.0
- Pixar USD version: 2024.02