Bump actions/checkout from 2 to 4 #124

Job	Run time
build (windows-latest, stable, build)	1m 1s
build (windows-latest, stable, test)	5m 48s
build (windows-latest, stable, clippy)	53s
build (windows-latest, nightly, build)	1m 25s
build (windows-latest, nightly, test)	4m 2s
build (windows-latest, nightly, clippy)	3m 26s
build (ubuntu-latest, stable, build)	37s
build (ubuntu-latest, stable, test)	2m 15s
build (ubuntu-latest, stable, clippy)	33s
build (ubuntu-latest, nightly, build)	48s
build (ubuntu-latest, nightly, test)	2m 16s
build (ubuntu-latest, nightly, clippy)	3m 0s
build (macos-latest, stable, build)	31s
build (macos-latest, stable, test)	1m 57s
build (macos-latest, stable, clippy)	23s
build (macos-latest, nightly, build)	52s
build (macos-latest, nightly, test)	2m 50s
build (macos-latest, nightly, clippy)	2m 54s
	35m 31s

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