Semi-automatic update. These checks were performed:
- built on NixOS
- found 3.1.1 with grep in /nix/store/4pydihqzddnsfmlc2zcn6qlwi7m2p1k1-nlohmann_json-3.1.1
- found 3.1.1 in filename of file in /nix/store/4pydihqzddnsfmlc2zcn6qlwi7m2p1k1-nlohmann_json-3.1.1
`cmake` should be in `nativeBuildInputs` as it is only required at build time. For obvious reasons we can't have the tests running during a cross-compile. I figured I'd update the package version while I was at it, though these changes have also been tested independently of the version update.
