Caching calls to download-to-store?

[Ricardo Wurmus]

Hi Guix,

the CRAN updater downloads every upstream archive three times.  The
importer memoizes calls to the download procedure, but one download that
is out of reach is the very first.

It is triggered by package-update/url-fetch in (guix upstream).  It uses
download-to-store, which will unconditionally fetch whatever URL it is
given.

I would very much like to parameterize download-to-store, so that we can
optionally cache the results of its invocation.

What do you think?

-- 
Ricardo