Bump cachix/cachix-action from 14 to 15 #1929

Job	Run time
Build Echidna on ubuntu-20.04	5m 18s
Build Echidna on macos-13	33m 22s
Build Echidna on windows-latest	27m 22s
Test with solc 0.4.25 on ubuntu-latest	4m 38s
Test with solc 0.5.7 on ubuntu-latest	3m 26s
Test with solc 0.6.12 on ubuntu-latest	3m 28s
Test with solc 0.7.5 on ubuntu-latest	3m 53s
Test with solc 0.4.25 on windows-latest	4m 8s
Test with solc 0.5.7 on windows-latest	3m 47s
Test with solc 0.6.12 on windows-latest	4m 43s
Test with solc 0.7.5 on windows-latest	5m 4s
	1h 39m 9s

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