Bump actions/checkout from 2 to 4 #254

Job	Run time
no_std	33s
miri	1m 18s
build (ubuntu, stable, locked, true)	11m 47s
build (windows, stable, locked, false)	23m 53s
build (macos, stable, locked, false)	16m 30s
build (ubuntu, nightly, locked, false)	11m 34s
build (ubuntu, beta, locked, false)	11m 43s
build (ubuntu, stable, latest, false)	12m 21s
build (ubuntu, stable, minimal, false)	11m 17s
fuzz	8m 14s
features	24m 58s
deploy	0s
	2h 14m 8s

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