Add `rust-toolchain.toml` to raytracing example #1491

Job	Run time
build_examples	5m 49s
build_nightly	2m 28s
Check	2m 2s
build_benchmarks	1m 45s
Verify that web-sys is compiled correctly	1m 27s
Clippy	7m 21s
dist_linux_aarch64_gnu	4m 58s
dist_linux_x86_64_musl	4m 16s
dist_macos_aarch64	4m 23s
dist_macos_x86_64	1m 44s
dist_windows	4m 25s
doc_api	1m 28s
doc_book	37s
Rustfmt	21s
Run js-sys crate tests	3m 17s
Build and test the deno example	1m 20s
Run native tests	2m 55s
Run wasm-bindgen crate tests with multithreading enabled	2m 20s
Test TypeScript output of wasm-bindgen	2m 10s
Run UI compile-fail tests	4m 57s
Run wasm-bindgen crate tests (unix)	7m 59s
Run web-sys crate tests	6m 30s
Run wasm-bindgen-webidl crate tests	4m 12s
test_examples	3m 25s
deploy	2m 33s
	1h 24m 42s

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