bug#26608: Provide --only-substitutes flag to "guix package --upgrade"

[Christopher Allan Webber]

Sometimes I do an upgrade and I want to get the latest security updates,
but I also am not really available to burn through a lot of cpu time,
(especially on my x200).

I'd love it if thre were a flag so that I could specify "only bother to
upgrade the packages where we only upgrade if a substitute is already
available.

Note that I looked at the source to see if this would be an easy thing
to add; I figured that this would be handled in roughly the same place
as --keep-going or --fallback, but it looks to me like those are
actually just passed over to the build daemon, so I'm not so sure how
easy it would be to patch this in while we're still using Nix's C
daemon?  I'm not sure.