Cargo headers are not properly installed.

Consumers of the library (e.g. scord) typically receive compilation errors:

/home/amiranda/repo/src/scord/rpc_server.cpp:30:10: fatal error: cargo/cargo.hpp: No such file or directory
   30 | #include <cargo/cargo.hpp>                                                             
      |          ^~~~~~~~~~~~~~~~~                                                             
compilation terminated.