Bump actions/checkout from 2 to 4 (#45) #129

Job	Run time
build (windows-latest, stable, build)	1m 36s
build (windows-latest, stable, test)	4m 59s
build (windows-latest, stable, clippy)	1m 1s
build (windows-latest, nightly, build)	1m 12s
build (windows-latest, nightly, test)	4m 26s
build (windows-latest, nightly, clippy)	52s
build (ubuntu-latest, stable, build)	18s
build (ubuntu-latest, stable, test)	2m 0s
build (ubuntu-latest, stable, clippy)	30s
build (ubuntu-latest, nightly, build)	39s
build (ubuntu-latest, nightly, test)	1m 57s
build (ubuntu-latest, nightly, clippy)	49s
build (macos-latest, stable, build)	22s
build (macos-latest, stable, test)	2m 0s
build (macos-latest, stable, clippy)	12s
build (macos-latest, nightly, build)	44s
build (macos-latest, nightly, test)	1m 37s
build (macos-latest, nightly, clippy)	20s
	25m 34s

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