Bump actions/download-artifact from 3 to 4 #1978

Job	Run time
conformance	3m 57s
build (2_12, mima)	1m 27s
build (2_12, e2e)	9s
build (2_12, test)	11m 15s
build (2_13, mima)	1m 29s
build (2_13, e2e)	4s
build (2_13, test)	10m 51s
build (3, mima)	7s
build (3, e2e)	4s
build (3, test)	11m 10s
build (2_12, examples_and_formatting)	3m 2s
build (2_13, scala3_compat_test)	7m 4s
build (3, scala3_compat_test)	8m 14s
ci-passed	1s
	58m 54s

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