Bump actions/download-artifact from 5.0.0 to 6.0.0 #281

Job	Run time
config	13s
build (iOS)	5m 52s
build (watchOS)	7m 29s
build (macOS)	37s
build (tvOS)	5m 36s
build (visionOS)	5m 28s
Briefcase testbed (macOS)	1m 14s
CPython testbed (tvOS)	2m 23s
CPython testbed (iOS)	3m 14s
CPython testbed (visionOS)	2m 25s
Briefcase testbed (iOS)	4m 24s
Cross-platform env test (x86_64-iphonesimulator)	17s
Cross-platform env test (arm64-iphonesimulator)	30s
Cross-platform env test (arm64-iphoneos)	14s
	39m 56s

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