[R-SIG-Mac] Mirror sync issues with macOS binaries for new package releases.

[Jim Hester]

Recently it seems to have become much more common for the PACKAGES*
metadata to get out of sync with the actual availability of macOS binaries.
e.g. the PACKAGES* metadata will show there is a binary available, but it
is not yet fully synced and users get download failures. We had 4 issues
(and one email) opened in various repositories related to this (about the
withr packages) yesterday.

https://github.com/r-lib/pak/issues/262
https://github.com/r-lib/actions/issues/141#issuecomment-769205048
https://github.com/r-lib/actions/issues/235
https://github.com/r-lib/withr/issues/160#issuecomment-769261447

This is also not an isolated incident with one package, the sf package had
the same thing happen, and I would not be surprised if there were more I am
aware of.

https://github.com/r-spatial/sf/issues/1586

This seems to happen sporadically, but recently has been occurring much
more frequently.

It also seems to only be a problem for the macOS binaries, Windows binaries
don't seem to have this issue. Either because they are synced differently
or because they are built in the same location as the main CRAN mirror.

Using `rsync --delay-updates` could possibly help to ensure more atomic
mirroring if it is not already being used.

I CC'd Kurt on this email as I am not sure he subscribes to R-Sig-Mac, and
he may be able to provide insight into this issue.

Jim

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