Fix weekly ci action

[fido-node]

Hey, I just wanted to create fresh install media, so I kept trying different approaches until it worked. Here are the changes I made:

• Changed the way to call the Guix binary with sudo (took this idea from guix-install-action).
• Replaced substitute URLs inside the guix-daemon unit file (not entirely happy with this solution, but it works).
• Added a rudimentary cache.

[fido-node]

Sorry for closing and opening it again. I've just spend some time to check that resulting image is bootable, and have a fresh nonguix kernel.

[fido-node]

Expect it to be a fix for: https://github.com/SystemCrafters/guix-installer/issues/27

[daviwil]

Thanks for trying to fix this! I had started working on a similar fix but never finished it. I'll merge this one and we'll see how it goes!