[bug#39684] [PATCH] etc: Automatically download the pgp key

[Julien Lepiller]

Hi guix,

this patch should improve the way the gpg key is fetched. Instead of
asking the user to copy the command, when the user passes
--allow-import-gpg, the script will import the gpg key by itself.

The rationale behind this is a user complaining a few weeks ago on the
Fediverse that the installation video didn't work. In fact, they didn't
understand the gpg command and failed to import the key; mentionning
--allow-import-gpg in the video should help in that case :)