[ci] fix flaky R CMD check test about library size

[jameslamb]

We now have two PRs hitting CI failures on R builds for this issue:

* checking installed package size ... NOTE
  installed size is  5.2Mb
  sub-directories of 1Mb or more:
    libs   4.5Mb

#3305 (comment), #3264 (comment)

That note is very unpredictable and likely to be allowed by CRAN. This PR proposes allowing it in CI jobs.

[jameslamb]

the check-docs failure looks legitimate

[jameslamb]

it seems research.microsoft.com is unavailable 😬

[github-actions]

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