Bump actions/checkout from 3 to 4 #1612

Job	Run time
Build Echidna on ubuntu-20.04	4m 5s
Build Echidna on macos-latest	3m 45s
Build Echidna on windows-latest	8m 48s
Test with solc 0.4.25 on ubuntu-latest	6m 37s
Test with solc 0.5.7 on ubuntu-latest	4m 34s
Test with solc 0.6.12 on ubuntu-latest	5m 22s
Test with solc 0.7.5 on ubuntu-latest	4m 59s
Test with solc 0.4.25 on windows-latest	4m 54s
Test with solc 0.5.7 on windows-latest	8m 25s
Test with solc 0.6.12 on windows-latest	9m 41s
Test with solc 0.7.5 on windows-latest	8m 15s
	1h 9m 25s

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