librt and libpthread were merged into libc with glibc 2.34, however, for compatibility, there exist empty .a files to satisfy the linker. But because Guix has a separate output for the static libraries, they need to be explicitly installed from ‘glibc:static’, which allows Cargo to do its thing.
Cargo itself should probably be fixed so that it no longer tries to link with these libraries, but until that time, the above work-around suffices.
