Bump checkout action #399

Job	Run time
build (stable, serde, ubuntu-latest)	27s
build (stable, serde, macos-latest)	4m 13s
build (stable, serde, windows-latest)	1m 54s
build (stable, base64, ubuntu-latest)	59s
build (stable, base64, macos-latest)	2m 57s
build (stable, base64, windows-latest)	2m 49s
build (stable, default, ubuntu-latest)	51s
build (stable, default, macos-latest)	4m 2s
build (stable, default, windows-latest)	2m 15s
build (nightly, serde, ubuntu-latest)	40s
build (nightly, serde, macos-latest)	2m 43s
build (nightly, serde, windows-latest)	1m 14s
build (nightly, base64, ubuntu-latest)	47s
build (nightly, base64, macos-latest)	4m 24s
build (nightly, base64, windows-latest)	1m 36s
build (nightly, simd_backend, ubuntu-latest)	49s
build (nightly, simd_backend, macos-latest)	2m 45s
build (nightly, default, ubuntu-latest)	44s
build (nightly, default, macos-latest)	2m 47s
build (nightly, default, windows-latest)	1m 9s
	40m 5s

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