[R] [CI] Add a crossbow job that mimics CRAN's old macOS

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Jeroen's autobrew does this using travis:

https://github.com/autobrew/homebrew-core/blob/high-sierra/.travis.yml

It would be good to test this on our own before the release process

Reporter: Jonathan Keane / @jonkeane
Assignee: Jacob Wujciak / @assignUser

PRs and other links:

• GitHub Pull Request #13925

_{Note: This issue was originally created as ARROW-15481. Please see the migration documentation for further details.}

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Jacob Wujciak / @assignUser:
Working on getting self-hosted 10.13 runners matching CRAN with r-release and r-oldrel for nightlies and other jobs (as-cran?) open for suggestions.

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Jacob Wujciak / @assignUser:
Technically this has been done with: #13769
but I want to add the 10.13 runners to the r-binary-packages job.

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Neal Richardson / @nealrichardson:
Issue resolved by pull request 13925
#13925