skein: bump `threefish` to v0.6.0-rc.0 (#769) #41

Job	Run time
set-msrv / set-msrv	3s
minimal-versions / minimal-versions	33s
test (stable)	28s
build (1.85.0, wasm32-unknown-unknown)	24s
test (1.85.0)	37s
build (stable, wasm32-unknown-unknown)	16s
build (1.85.0, thumbv7em-none-eabi)	22s
build (stable, thumbv7em-none-eabi)	17s
	3m 0s

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