bug#22937: guix package fails when --substitute-urls specifies an HTTPS endpoint

[Ludovic Courtès]

After some more testing and tweaks, notably with
<https://hydra.gnunet.org> and its brand new Let’s Encrypt certificate,
I pushed the patch as 9b7bd1b160be7c740a798c09e3b8986368b92036.

I can no longer reproduce the ECONNRESET issue I mentioned for
<https://hydra-mirror.marusich.info>, but feel free to open a new bug if
that comes up again!

For the record, the main test I used for that was:

  $ sudo rm -rf /var/guix/substitute/cache
  $ guix build libreoffice --no-grafts -n \
        --substitute-urls=https://hydra.gnunet.org

That forces a redownload of all .narinfo files from hydra.gnunet.org,
which involves HTTP pipelining.

Anyway, thanks for giving me an incentive to address this.  This was
long overdue!

Ludo’.