rustup: update to `nightly-2023-09-30`. #2912

Job	Run time
Lint	2m 48s
cargo-deny	2m 41s
Test (ubuntu-20.04-16core, x86_64-unknown-linux-gnu)	3m 29s
Test (windows-2022-16core, x86_64-pc-windows-msvc)	4m 48s
Test (macOS-latest-xl, x86_64-apple-darwin)	5m 40s
Test (ubuntu-20.04-16core, aarch64-linux-android)	4m 2s
	23m 28s

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