Bump actions/cache from 2 to 4 (#44) #127

Job	Run time
build (windows-latest, stable, build)	3m 13s
build (windows-latest, stable, test)	5m 14s
build (windows-latest, stable, clippy)	2m 27s
build (windows-latest, nightly, build)	1m 20s
build (windows-latest, nightly, test)	4m 29s
build (windows-latest, nightly, clippy)	52s
build (ubuntu-latest, stable, build)	22s
build (ubuntu-latest, stable, test)	2m 12s
build (ubuntu-latest, stable, clippy)	1m 3s
build (ubuntu-latest, nightly, build)	44s
build (ubuntu-latest, nightly, test)	1m 59s
build (ubuntu-latest, nightly, clippy)	41s
build (macos-latest, stable, build)	34s
build (macos-latest, stable, test)	2m 4s
build (macos-latest, stable, clippy)	14s
build (macos-latest, nightly, build)	33s
build (macos-latest, nightly, test)	2m 3s
build (macos-latest, nightly, clippy)	21s
	30m 25s

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