[v1.8.x] Backport CI fixes (#19377)

[josephevans]

Backport #19377 to v1.8.x

• Update repo and key used for installing r. Should fix sanity builds in v1.x.

• Use correct repo (trusty) as v1.x branch uses ubuntu:16.04 for container. Also remove copy GPG key command.

• Remove all references to old expired key.

Co-authored-by: Joe Evans joeev@amazon.com

Description

(Brief description on what this PR is about)

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
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