[BUILD] NanoVDB - OptiX Root Directory

## Environment
**Operating System:** Ubuntu 20.04
**Version / Commit SHA:** feature/nanovdb
**CMake Version:** 3.18
**Compiler:** GCC 9

### Describe the problem

Can't find the OptiX root directory
```sh
$ cmake ..
/// etc ...
CMake Warning at cmd/CMakeLists.txt:235 (message):
- OptiX not found for Viewer. Removing OptiX support.
```

I've checked the FindOptix.cmake that's in the repo and I found that the variable `${Optix_Root}` is being used but not exposed to the user through cmake:
https://github.com/AcademySoftwareFoundation/openvdb/blob/9b79bb0dd66a442149083c8093deefcb03f881c3/nanovdb/cmake/FindOptiX.cmake#L26

If it helps it's quite easy to fix in Linux, just run this and it works

```sh
$ cmake -DOptiX_ROOT= ..
```

I know this feature branch is still a work in progress, so I'm not complaining :angel: I'm just raising the issue in case any other developer is seeing the same issue.

I'm happy to expose this flag to the build system if you are accepting PR's

Start with nanovdb/cmake/FindOptiX.cmake around line 26 and inspect how the OptiX root directory is discovered. Reproduce the Ubuntu CMake configure shown in the issue, then verify that the documented OptiX root variable is accepted and OptiX support is retained when configuring the Viewer.

Written by the indexing model from the issue text.