Add `rust-toolchain.toml` to raytracing example #1395

Job	Run time
build_examples	3m 10s
build_benchmarks	1m 54s
Check	1m 32s
build_nightly	3m 8s
Verify that web-sys is compiled correctly	1m 23s
Clippy	2m 58s
dist_linux_aarch64_gnu	3m 6s
dist_linux_x86_64_musl	3m 7s
dist_macos_aarch64	2m 57s
dist_macos_x86_64	3m 0s
dist_windows	2m 55s
doc_api	1m 35s
doc_book	35s
Rustfmt	11s
Run js-sys crate tests	2m 54s
Build and test the deno example	1m 20s
Run native tests	2m 39s
Run wasm-bindgen crate tests with multithreading enabled	2m 4s
Test TypeScript output of wasm-bindgen	1m 21s
Run wasm-bindgen crate tests (unix)	3m 0s
Run UI compile-fail tests	2m 48s
Run web-sys crate tests	2m 22s
Run wasm-bindgen-webidl crate tests	1m 39s
test_examples	1s
deploy	1s
	51m 40s

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