GHA build

Use v4 of the artifact actions, since v2 is disallowed and v3 is depr… #62

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Run time

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Job	Run time
7.0 Ub-20 gcc-9	7m 17s
3.15 MacOS clang-12	2m 8s
3.15 Win VS2019, static	7m 58s
	17m 23s