Bump `rand` to v0.10.0-rc.8 (#2193) #2506

Job	Run time
build (stable, wasm32-unknown-unknown)	29s
build (stable, thumbv7em-none-eabi)	25s
build (1.85.0, wasm32-unknown-unknown)	25s
test (1.85.0)	1m 12s
test (stable)	1m 8s
build (1.85.0, thumbv7em-none-eabi)	30s
minimal-versions	0s
	4m 9s

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