Bump actions/download-artifact from 4.1.5 to 4.1.8 #24

Job	Run time
Ubuntu Build release	5m 49s
Ubuntu Build debug	1m 43s
Ubuntu Build asan	2m 47s
Ubuntu Build msan	6m 25s
Ubuntu Build tsan	2m 31s
Ubuntu Build scalar	2m 0s
Ubuntu Build lowprecision	4m 43s
Ubuntu Build coverage	40s
Ubuntu Build release-nojpeg	1m 52s
Ubuntu Build release-lcms2	2m 27s
Ubuntu Build release-system-lcms2	2m 0s
Ubuntu Build release:minimal	1m 49s
Ubuntu Build release:gcc8	1m 54s
Ubuntu Build release:clang-7	2m 10s
MacOS Build release:osx	1m 31s
	40m 21s

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