Add clarifying comment about stable Rust toolchain usage #576

Job	Run time
build-and-test (x64, windows-latest, x64)	2m 10s
build-and-test (x86, windows-latest, x64_x86)	1m 0s
build-and-test (arm64, windows-11-arm, arm64)	1m 37s
	4m 47s

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