Bump actions/download-artifact from 4 to 6 #276

Job	Run time
Build sdist	28s
Build data (basemap_data)	31s
Build data (basemap_data_hires)	58s
Build wheels (macos-13)	1m 6s
Build wheels (ubuntu-22.04)	1m 17s
Build wheels (windows-2022)	1m 6s
Build wheels (macos-14)	1m 2s
Build docs	0s
Deploy docs	0s
Upload packages	0s
Check packages	0s
	6m 28s

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