Bump actions/checkout from 4 to 6 (#109) #304

Job	Run time
doc	25s
rustfmt	6s
test (stable)	33s
clippy	27s
cross (powerpc-unknown-linux-gnu, stable)	1m 3s
cross (powerpc-unknown-linux-gnu, 1.85.0)	1m 2s
test (1.85.0)	39s
no_std (stable, thumbv7em-none-eabi)	36s
no_std (1.85.0, thumbv7em-none-eabi)	41s
no_std (1.85.0, wasm32-unknown-unknown)	40s
no_std (stable, armv7a-none-eabi)	39s
no_std (stable, wasm32-unknown-unknown)	38s
no_std (1.85.0, armv7a-none-eabi)	42s
	8m 11s

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