Bump dawidd6/action-download-artifact from 3 to 6 in /.github/workflows #1780

Job	Run time
validate	50s
docs / Build linux64	1m 9s
test / Windows MSVC / Build win64	17m 52s
test / Linux gcc-10 / Build linux64	5m 44s
test / Linux gcc-12 / Build linux64	7m 38s
test / Run Linux test suite (gcc-12, all plugins) / Test (ubuntu, gcc-12, all plugins, default config)	1m 42s
test / Run Linux test suite / Test (ubuntu, gcc-10, default plugins, default config)	1m 35s
test / Run Windows test suite / Test (windows, msvc, default plugins, default config)	1m 9s
test / Run Windows test suite (empty config) / Test (windows, msvc, default plugins, empty config)	1m 14s
	38m 53s

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