This manual page

https://www.gnu.org/software/guix/manual/html_node/Substitute-Server-Authorization.html#Substitute-Server-Authorization

says that I should loo for substitutes authorization keys in

prefix/share/guix/hydra.gnu.org.pub

I'm on a laptop with GuixSD on it and I don't understand: which is my prefix ?

I'm asking this because Efraim invited me to use his substitute server on the irc channel and I don't know how to set it up

I want to use his substitute server because I'd like to build the installation image from the branch with the graphical installer, in order to test it in a visrtual machine

But when I tried to build the installation image I got this (tl;dr linux-libre-4.13.2 missing)

~/projects/graphical-installer_guix [env]$ ./pre-inst-env guix system disk-image gnu/system/install.scm
guix system: warning: Your Guix installation is 17 days old.
guix system: warning: Consider running 'guix pull' followed by
'guix system reconfigure' to get up-to-date packages and security updates.

substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From http://linux-libre.fsfla.org/pub/linux-libre/releases/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz...
ERROR: download failed "http://linux-libre.fsfla.org/pub/linux-libre/releases/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz" 404 "Not Found"

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From ftp://alpha.gnu.org/gnu/guix/mirror/linux-libre-4.13.2-gnu.tar.xz...
ERROR: Throw to key `ftp-error' with args `(#<input-output: socket 15> "RETR linux-libre-4.13.2-gnu.tar.xz" 550 "Failed to open file.\r")'.

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From https://ftpmirror.gnu.org/gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz...
following redirection to `https://mirror2.mirror.garr.it/mirrors/gnuftp/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz'...
ERROR: download failed "https://mirror2.mirror.garr.it/mirrors/gnuftp/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz" 404 "Not Found"

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From ftp://ftp.cs.tu-berlin.de/pub/gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz...
ERROR: In procedure connect*: Connection timed out

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From ftp://ftp.funet.fi/pub/mirrors/ftp.gnu.org/gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz...
ERROR: Throw to key `ftp-error' with args `(#<input-output: socket 19> "CWD 4.13.2-gnu" 550 "Can't change directory to 4.13.2-gnu: No such file or directory\r")'.

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From http://ftp.gnu.org/pub/gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz...
ERROR: download failed "http://ftp.gnu.org/pub/gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz" 404 "Not Found"

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From http://mirror.hydra.gnu.org/file/linux-libre-4.13.2-gnu.tar.xz/sha256/166yy7nah2h2ffxqgb92nfwrvihna3kvdx4ryppf34gmybmmfw36...
ERROR: download failed "http://mirror.hydra.gnu.org/file/linux-libre-4.13.2-gnu.tar.xz/sha256/166yy7nah2h2ffxqgb92nfwrvihna3kvdx4ryppf34gmybmmfw36" 404 "Not Found"

Starting download of /gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz
From http://tarballs.nixos.org/sha256/166yy7nah2h2ffxqgb92nfwrvihna3kvdx4ryppf34gmybmmfw36...
ERROR: download failed "http://tarballs.nixos.org/sha256/166yy7nah2h2ffxqgb92nfwrvihna3kvdx4ryppf34gmybmmfw36" 404 "Not Found"
failed to download "/gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz" from ("http://linux-libre.fsfla.org/pub/linux-libre/releases/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz" "ftp://alpha.gnu.org/gnu/guix/mirror/linux-libre-4.13.2-gnu.tar.xz" "mirror://gnu/linux-libre/4.13.2-gnu/linux-libre-4.13.2-gnu.tar.xz")
builder for `/gnu/store/9nkcdrfjn0p8lqiy20myv65nc48mpxc2-linux-libre-4.13.2-gnu.tar.xz.drv' failed to produce output path `/gnu/store/i1h3wfag2j71xz1vnkk6lhsbymgrdp8a-linux-libre-4.13.2-gnu.tar.xz'
cannot build derivation `/gnu/store/r931q9wwrad9sm2l6r6ih2crqa87bxw5-linux-libre-4.13.2-gnu.tar.xz.drv': 1 dependencies couldn't be built
cannot build derivation `/gnu/store/3ls60p58nqgiif4bxndzp5dxab1nc3jm-linux-libre-4.13.2.drv': 1 dependencies couldn't be built
guix system: error: build failed: build of `/gnu/store/3ls60p58nqgiif4bxndzp5dxab1nc3jm-linux-libre-4.13.2.drv' failed