Bump actions/download-artifact to @v8 #131

Job	Run time
build_wheel_and_sdist	15s
run_speed_tests (3.13, ubuntu-24.04)	15s
run_speed_tests (3.9, windows-latest)	1m 19s
run_speed_tests (3.13, windows-latest)	46s
run_speed_tests (3.9, ubuntu-24.04)	21s
	2m 56s

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