mark cmake-built libprotobuf's broken on unix

[h-vetinari]

There's some fall-out from building libprotobuf via cmake, see post-merge comments in conda-forge/libprotobuf-feedstock#128.

It's bizarre to me because the SO-numbers match (see this comment), but mark it broken for now to not further break people.

Original reporter (@peastman) mentioned specifically that:

This only happens on x86, not ARM.

Queries used:

mamba repoquery search libprotobuf=3.20.1=*_2 -p osx-64 --json | grep url | awk -F'conda-forge/' '{print $2}'  | awk -F'",' '{print $1}
mamba repoquery search libprotobuf=3.21.5=*_1 -p osx-64 --json | grep url | awk -F'conda-forge/' '{print $2}'  | awk -F'",' '{print $1}

and so on for the other platforms