Bump actions/download-artifact from 3 to 4 (#1621) #1983

Job	Run time
build (2_12, mima)	1m 5s
conformance	4m 4s
build (2_12, e2e)	5s
build (2_12, test)	11m 12s
build (2_13, mima)	1m 6s
build (2_13, e2e)	8s
build (2_13, test)	10m 49s
build (3, mima)	8s
build (3, e2e)	8s
build (3, test)	9m 11s
build (2_12, examples_and_formatting)	2m 47s
build (2_13, scala3_compat_test)	5m 52s
build (3, scala3_compat_test)	9m 34s
ci-passed	1s
	56m 10s

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