Better location to download OptiX headers

[jfpanisset]

Currently

OpenShadingLanguage/src/build-scripts/gh-installdeps.bash

Line 153 in 96d0131

OPTIXLOC=https://developer.download.nvidia.com/redist/optix/v${OPTIX_VERSION}

downloades OptiX headers from https://developer.download.nvidia.com/redist/optix/... which is an ad-hoc location that's often behind in supported versions.

There is now an official GitHub repo which has downloads for every version of the OptiX headers from 7.0 to 9.0:

https://github.com/NVIDIA/optix-dev

[lgritz]

I think this was closed by #1992