Building and caching old Guix derivations for a faster time machine

[Ricardo Wurmus]

Hi Guix,

to me the biggest downside of using “guix time-machine” is that it has
to do a lot of boring work before the interesting work begins.  The
boring work includes building Guix derivations for the given channels,
most of which have long been collected as garbage on ci.guix.gnu.org.

It would be helpful, I think, to more aggressively cache these
derivations and their outputs, and to go back in time and build the
derivatinons for past revisions of Guix.  I would expect there to be a
lot of overlap in the produced files, so perhaps it won’t cost all that
much in terms of storage.

What do you think?

-- 
Ricardo