ci: bump actions/checkout to v6 #993

Job	Run time
minimal-versions / minimal-versions	29s
test (stable)	20s
build (stable, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	19s
build (1.85.0, wasm32-unknown-unknown)	19s
test (1.85.0)	23s
build (1.85.0, thumbv7em-none-eabi)	22s
	2m 32s

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