bug#22774: Bootstrapping issue with HTTPS downloads

[Ludovic Courtès]

Hi!

I pushed the whole patch series with the fix as
05ceb8dcaf480a47cddf94ac979070b76df6556c.

The final patch series uses a new internal helper, ‘guix
perform-download’, instead of trying to bend ‘guix download’ to these
specific needs.

Ludo’.