Fix CI builds failing due to invalid GPG keys.

[josephevans]

Description

We are having CI failures due to a GPG key expiring, which results in packages not being installed properly and our docker build scripts failing. This PR updates the key and repository to the latest.

[2020-10-19T18:51:21.002Z] Reading package lists...
[2020-10-19T18:51:21.002Z] W: GPG error: http://cran.rstudio.com/bin/linux/ubuntu trusty/ Release: The following signatures were invalid: KEYEXPIRED 1602869253 KEYEXPIRED 1602869253 KEYEXPIRED 1602869253
[2020-10-19T18:51:21.002Z] W: The repository 'http://cran.rstudio.com/bin/linux/ubuntu trusty/ Release' is not signed.
[2020-10-19T18:51:21.002Z] W: GPG error: https://cran.rstudio.com/bin/linux/ubuntu xenial/ InRelease: The following signatures were invalid: KEYEXPIRED 1602869253 KEYEXPIRED 1602869253 KEYEXPIRED 1602869253
[2020-10-19T18:51:21.002Z] W: The repository 'https://cran.rstudio.com/bin/linux/ubuntu xenial/ InRelease' is not signed.
[2020-10-19T18:51:21.002Z] E: Failed to fetch store:/var/lib/apt/lists/partial/cran.rstudio.com_bin_linux_ubuntu_trusty_Packages.gz Hash Sum mismatch
[2020-10-19T18:51:21.002Z] E: Some index files failed to download. They have been ignored, or old ones used instead.
[2020-10-19T18:51:21.002Z] + wget -qO /usr/lib/llvm-6.0/share/clang/run-clang-tidy.py https://raw.githubusercontent.com/llvm-mirror/clang-tools-extra/7654135f0cbd155c285fd2a37d87e27e4fff3071/clang-tidy/tool/run-clang-tidy.py
[2020-10-19T18:51:21.002Z] /usr/lib/llvm-6.0/share/clang/run-clang-tidy.py: No such file or directory
[2020-10-19T18:51:21.258Z] The command '/bin/sh -c /work/ubuntu_clang.sh' returned a non-zero code: 1

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @josephevans , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

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CI supported jobs: [edge, windows-gpu, unix-cpu, clang, centos-cpu, website, unix-gpu, sanity, windows-cpu, miscellaneous, centos-gpu]

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Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[josephevans]

Relevant post: https://www.bentoh.my/post/gpg-key-problem/

[ChaiBapchya]

Thanks a lot Joe!