Add rust-toolchain.toml to raytracing example

[jthemphill]

The raytracing example requires a nightly build of Rust, and the source code to the Rust std library.

Running build.sh while your stable toolchain is active will result in an error:

error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel

I tried changing build.sh to run cargo +nightly, but this broke CI.

My inclination here is to describe these dependencies with a toolchain file. It's one less build flag to forget.

[daxpedda]

Thank you, this is great!

[daxpedda]

CI is failing because of the toolchain file.
I just took a quick look at the documentation to see if there is some flag to ignore it but couldn't really find anything. Would you mind exploring what to do here?

A quick solution that comes to mind here is to just use +stable to override the toolchain file.

[jthemphill]

Okay, I do think the clippy CI run needs to either install a nightly version of clippy, or it needs to change the command from cargo clippy to cargo +stable clippy to specify that we use stable clippy even when arust-toolchain.toml file calls for nightly.

[daxpedda]

cargo +stable clippy

I'm in favor of that.

[daxpedda]

Thanks.