Install OptiX on test runner machines to allow enabling ray tracing tests in CI

[ksavoie-nv]

Slang needs a copy of OptiX at runtime to compile OptiX shaders using nvrtc, but the CI machines don't have it, causing ray tracing tests to fail.

Can we install OptiX 8.0 on the test machines?

Alternatively, we could add a way to tell Slang where to find the OptiX headers (maybe via an environment variable like OPTIX_PATH?) and have the CI script fetch them before executing the tests.

[cheneym2]

When this is fixed, we can enable "optix" category tests on CI, whether explicitly in the workflow yaml file, or by default as talked about here: #6404 (comment)

However the error symptom of running optix slang-tests without available optix sdk is unintuitive right now. Instead of disabling the optix tests as unsupported, or even showing an error related to the optix installation, it just fails the related tests with incorrect output, so that probably makes it undesirable to enable the 'optix' category by default unless slang-test can skip optix tests gracefully when unsupported.

[csyonghe]

we are currently looking to migrate out of our self-hosted machines for CI. let's wait until we have a new plan for our CI setup.