Substitute caching restored

[Ludovic Courtès]

Hello Guix!

Tired of updating the list of substitutes?  I have some good news for
you!

Since the switch to Guile 2.2.2, because of a bug (an otherwise known
bug) in SRFI-19, ‘guix substitute’ would be storing the wrong date in
cached narinfos in /var/guix/substitute/cache.  Consequently, we’d be
updating most narinfos as soon as we had cached them!

Commit b547349d505c57fd679b6e48c472d8ab65469c96 fixes that and
c96ed00910d9238a43469e5f00a8261253294257 updates the ‘guix’ package.

If you’re on GuixSD, ‘guix system reconfigure’ will give you the fix.
If you’re on a foreign distro and you used the binary installation
method, run “guix pull && guix package -u guix” as root.

Ludo’.

PS: No that doesn’t solve <https://bugs.gnu.org/22990>, so we’re still
    updating the list of substitutes more than we’d like…