Bump dawidd6/action-download-artifact from 2 to 6 in /.github/workflows #355

Job	Run time
build (ubuntu-20.04, 3.9.9)	6m 2s
build (ubuntu-20.04, 3.10.6)	5m 59s
coverage	2m 40s
test-python (ubuntu-20.04, 3.9.9)	2m 1s
lint-files	23s
test-python (ubuntu-20.04, 3.10.6)	5m 29s
check-version-and-changelog	14s
build-conda	3m 3s
check-for-functional-changes	0s
deploy	0s
publish-to-conda	0s
test-on-windows	3m 39s
	29m 30s

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