Allow disabling Slither #2087

Job	Run time
Build Echidna on ubuntu-20.04	22m 26s
Build Echidna on macos-13	45m 8s
Build Echidna on windows-latest	30m 15s
Test with solc 0.4.25 on ubuntu-latest	3m 13s
Test with solc 0.5.7 on ubuntu-latest	3m 39s
Test with solc 0.6.12 on ubuntu-latest	3m 33s
Test with solc 0.7.5 on ubuntu-latest	4m 7s
Test with solc 0.4.25 on windows-latest	3m 14s
Test with solc 0.5.7 on windows-latest	5m 54s
Test with solc 0.6.12 on windows-latest	6m 6s
Test with solc 0.7.5 on windows-latest	6m 19s
	2h 13m 54s

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