fix nix develop

with this fix, on nixos you can build the project by doing:

```shell
$ nix develop # ensure dependencies to build
$ cargo build # succesfully build
```

otherwise there is an error in building rocksdb

flake.nix

@@ -79,6 +79,7 @@
 
           devShells.default = mkShell {
             inputsFrom = [ bin ];
+            LIBCLANG_PATH = "${pkgs.libclang.lib}/lib"; # for rocksdb
           };
         }
       );