R tests broken - mran.microsoft.com retired

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Sometime between between May 31 and Jun 4, our test suite started failing for the R buildpack.

No snapshot found for 2016-01-03 or 7 days prior in mran.microsoft.com

I figure its a regression caused by mran.microsoft.com no longer providing snapshots that old.

Or ooops, based on https://mran.microsoft.com/timemachine, its worse, its being shut down.

The banner links to this announcement.

After January 31, 2023, we will no longer maintain the CRAN Time Machine snapshots. Existing snapshots will be available until June 30, 2023.

Recommended Action:
You should plan to migrate from the use of Microsoft R Application Network website and CRAN Time Machine to avoid any interruptions. The CRAN Time Machine functionality can be replaced by using the miniCRAN R package as documented. The use of “Explore Packages” functionality from MRAN can be replaced by CRAN Task Views.

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Cross posting @cboettig's comment #909 (comment):

@yuvipanda MRAN has is slated for imminent retirement July 1, and may not still be doing any snapshorts?

I recommend switching over to Posit package manager, which has snapshots (and binary builds! though it goes back only to Oct 2017; MRAN dated back to 2014).