Fix importer workflow

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The importer workflow has been broken since Feb 27 because actions/cache@v2 was discontinued! Probably a 1-character fix for now (changing the actions/cache version number from 2 to 4, since they take the same options), but it's also worth noting that Magic Nix Cache runs on the same legacy Cache API as actions/cache@v2 and it's not clear how much longer it'll continue to be supported on GitHub Enterprise.