bug#47734: Guix install script on foreign distro fails to add berlin public key

[Julien Lepiller]

Hi Guix!

When installing Guix on a new machine (foreign distro), substitute were not 
properly set up:

…
Permit downloading pre-built package binaries from the project's build farm? 
(yes/no) yes
/home/roptat/guix-install.sh: line 445: 
/root/.config/guix/current/share/guix/ci.guix.gnu.org.pub: No such file or 
directory

That was the latest version of the script downloaded from savannah. It 
downloaded and installed the 1.2.0 tarball.

I was able to fix that later manually, but let's make sure the script doesn't 
fail like that with the release :)