Closes #30612

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/30612 from k0T0z:correct-cmake-zlib-library-var-name 2c85c6bf3ab3ef1e756e5ebcf0f991ba5afe9e52
PiperOrigin-RevId: 999181307