protobuf version mismatch on CI

[leezu]

[2020-12-02T23:35:43.300Z] [libprotobuf FATAL google/protobuf/stubs/common.cc:78] This program was compiled against version 2.6.1 of the Protocol Buffer runtime library, which is not compatible with the installed version (3.0.0). Contact the program author for an update. If you compiled the program yourself, make sure that your headers are from the same version of Protocol Buffers as your link-time library. (Version verification failed in "/io/onnx/.setuptools-cmake-build/onnx/onnx_ONNX_REL_1_7-ml.pb.cc".)

During compile: https://jenkins.mxnet-ci.amazon-ml.com/blue/organizations/jenkins/mxnet-validation%2Funix-cpu/detail/PR-17794/17/pipeline
During test runtime: https://jenkins.mxnet-ci.amazon-ml.com/blue/organizations/jenkins/mxnet-validation%2Funix-gpu/detail/PR-17794/10/pipeline

Looks like #19588 fixes the issue. But other PRs modifying the Dockerfiles are affected.

[leezu]

The issue persists after the upgrade to 20.04

[2020-12-04T03:42:24.967Z] [libprotobuf FATAL google/protobuf/stubs/common.cc:79] This program was compiled against version 2.6.1 of the Protocol Buffer runtime library, which is not compatible with the installed version (3.6.1).  Contact the program author for an update.  If you compiled the program yourself, make sure that your headers are from the same version of Protocol Buffers as your link-time library.  (Version verification failed in "google/protobuf/descriptor.pb.cc".)

[2020-12-04T03:42:24.967Z] terminate called after throwing an instance of 'google::protobuf::FatalException'

[2020-12-04T03:42:24.967Z]   what():  This program was compiled against version 2.6.1 of the Protocol Buffer runtime library, which is not compatible with the installed version (3.6.1).  Contact the program author for an update.  If you compiled the program yourself, make sure that your headers are from the same version of Protocol Buffers as your link-time library.  (Version verification failed in "google/protobuf/descriptor.pb.cc".)

https://jenkins.mxnet-ci.amazon-ml.com/blue/organizations/jenkins/mxnet-validation%2Funix-cpu/detail/PR-17794/18/pipeline/

The question is why does onnx introduce a dependency on protobuf 2.6.1 and why only in such a flaky manner.