nix: static builds and release workflow #1696

Job	Run time
Build Echidna on ubuntu-20.04	4m 42s
Build Echidna on macos-latest	3m 20s
Build Echidna on windows-latest	12m 11s
Test with solc 0.4.25 on ubuntu-latest	4m 8s
Test with solc 0.5.7 on ubuntu-latest	6m 1s
Test with solc 0.6.12 on ubuntu-latest	4m 29s
Test with solc 0.7.5 on ubuntu-latest	5m 17s
Test with solc 0.4.25 on windows-latest	6m 9s
Test with solc 0.5.7 on windows-latest	7m 2s
Test with solc 0.6.12 on windows-latest	7m 49s
Test with solc 0.7.5 on windows-latest	9m 13s
	1h 10m 21s

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