Add arnold/plugins from HOUDINI_PATH

**Describe the feature you'd like**
We need to add the arnold/plugins subfolders in each of the directories specified by the HOUDINI_PATH environment variable.

**Describe the solution you'd like**
We can reuse code from the volume that loads the htoa libraries.

Locate the volume that loads the htoa libraries and trace how it handles the directories in HOUDINI_PATH. Then determine how the arnold/plugins subfolder should be added for each directory. Done means those plugin subfolders are included wherever the environment variable specifies a path.

Written by the indexing model from the issue text.