ci: bump actions/checkout to v6 (#545) #994

Job	Run time
test (1.85.0)	22s
test (stable)	20s
build (1.85.0, thumbv7em-none-eabi)	19s
build (stable, thumbv7em-none-eabi)	20s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	30s
	2m 28s

Provide feedback