CRAN doesn't allow us to use rappdirs::user_cache_dir()

[mrcaseb]

With v1.0.3 we released code to CRAN that creates a user cache directory using rappdirs. We received this lovely mail a couple days later

This uses ~/.cache/nfl4th (and /Users/ripley/Library/Caches/nfl4th on
macOS). That is prohibited under the CRAN policy with which you have
just warranted compliance.
The package is now scheduled for archival on 2023-08-29, and this will
necessitate archiving its CRAN strong reverse dependency, nflverse.
--
Brian D. Ripley, ripley@stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford

This is a known issue, see r-lib/rappdirs#27 and r-lib/rappdirs#40
So now we are forced to switch over to tools:: R_user_dir() which is problematic because it was introduced with R4.0.0.
The solution is to use the backports package with R versions < 4.0

Fun fact: even the freakin .Rd file of tools:: R_user_dir() claims, that rappdirs works fine.