ci: actions-checkout: v4 -> v6 #333

Job	Run time
Format	16s
test	43s
Clippy	23s
check (thumbv7em-none-eabihf, nightly)	22s
check (thumbv6m-none-eabi, nightly)	21s
rustdoc	16s
check (thumbv6m-none-eabi, stable)	19s
check (thumbv7em-none-eabihf, stable)	16s
	2m 56s

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