Bump actions/checkout from 2 to 4 #125

Job	Run time
build (windows-latest, stable, build)	1m 52s
build (windows-latest, stable, test)	5m 40s
build (windows-latest, stable, clippy)	56s
build (windows-latest, nightly, build)	1m 37s
build (windows-latest, nightly, test)	4m 35s
build (windows-latest, nightly, clippy)	3m 34s
build (ubuntu-latest, stable, build)	40s
build (ubuntu-latest, stable, test)	2m 6s
build (ubuntu-latest, stable, clippy)	29s
build (ubuntu-latest, nightly, build)	52s
build (ubuntu-latest, nightly, test)	2m 6s
build (ubuntu-latest, nightly, clippy)	3m 13s
build (macos-latest, stable, build)	32s
build (macos-latest, stable, test)	1m 39s
build (macos-latest, stable, clippy)	29s
build (macos-latest, nightly, build)	3m 2s
build (macos-latest, nightly, test)	1m 53s
build (macos-latest, nightly, clippy)	31s
	35m 46s

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