ci: bump actions/checkout from 5 to 6 (#158) #330

Job	Run time
clippy	20s
careful	57s
doc	12s
build (1.85.0, thumbv7em-none-eabi)	48s
build (1.85.0, armv7a-none-eabi)	52s
build (stable, wasm32-unknown-unknown)	46s
test (stable)	34s
build (1.85.0, wasm32-unknown-unknown)	50s
build (stable, thumbv7em-none-eabi)	48s
build (stable, armv7a-none-eabi)	1m 5s
test (1.85.0)	35s
miri (x86_64-unknown-linux-gnu)	42s
miri (s390x-unknown-linux-gnu)	59s
rustfmt	10s
	9m 38s

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