While testing Nicolas's patch "Update giac-xcas", I found that `guix
download` accepts expired TLS certificates.

I tried visiting the upstream site in order to verify the hash of the
updated package, and my browsers (Firefox and Chromium) warned me that
the site's certificate had expired ~1 day ago.

However, `guix build -S` did not warn me or prevent me from downloading
the source code.

Perhaps it doesn't matter for the case of `guix build -S`, since we
already know what we expect to download. But, for `guix download`, this
is a bug.

[0]
http://lists.gnu.org/archive/html/guix-devel/2016-09/msg01460.html