Downgrade Jenkins agent to CUDA 9.2, to fix broken NCCL2 installation

[hcho3]

See https://xgboost-ci.net/blue/organizations/jenkins/xgboost/detail/PR-4231/2/pipeline/51.

The current scripts downloads NCCL2 from URL https://developer.download.nvidia.com/compute/redist/nccl/v2.2/nccl_2.2.13-1%2Bcuda${CUDA_VERSION}_x86_64.txz. This URL works fine when CUDA_VERSION is 8.x or 9.x but fails for 10.1.

TODO. Either find a link that's not behind a login wall, OR upload installers to a private S3 bucket.
EDIT. I think it would be best if we place NCCL2 installers inside the worker image. This way, we don't generate unnecessary HTTP traffic.

[codecov-io]

Codecov Report

Merging #4232 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4232   +/-   ##
=======================================
  Coverage   67.28%   67.28%           
=======================================
  Files         132      132           
  Lines       12220    12220           
=======================================
  Hits         8222     8222           
  Misses       3998     3998

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[hcho3]

I created a new issue to discuss improvements to Jenkins: #4234. I'd like to create a major refactor of Jenkins, for the sake of simplicity, de-duplication, and wider coverage.

[RAMitchell]

Hmmm we can probably get the new nccl binary up soon. Suggest merging this though so we can continue to use CI, we can change it back to 10.1 later.

[trivialfis]

Now that nccl is open sourced, can we compile it ourselves? I'm doing this for 10.1 setting

[rongou]

That's a very old version of nccl. Maybe upgrade to a newer version?

[hcho3]

Merging for now to unblock CI. Will follow up with another PR to support 10.1 properly