update build-installer to work on non guix systems

scripts/build-installer

@@ -1,6 +1,27 @@
 #!/bin/sh
 retry="$(dirname $0)/guix-retry"
 repo="$(realpath "$(dirname $0)/..")"
+extra_args=""
+
+if [ ! -d /gnu/store ]; then
+# we are likely in a non guix system
+# just for building the installer
+# make sure to authorize the substitute
+# servers for nonguix and sakura
+guix archive --authorize < "$repo/files/keys/nonguix.pub"
+guix archive --authorize < "$repo/files/keys/sakura.pub"
+substitutes=$((cat <<EOF
+https://substitutes.nonguix.org
+https://substitutes.freya.cat
+https://bordeaux.guix.gnu.org
+https://ci.guix.gnu.org
+EOF
+) | tr '\n' ' ')
+extra_args="--substitute-urls=\"$substitutes\""
+fi
+
+command="\"$retry\" guix time-machine -C \"$repo/channels.scm\" $extra_args -- system -L \"$repo/modules\" image -t iso9660 \"$repo/systems/installer.scm\""
+image=$(sh -c "$command")
+
 date="$(date -Iseconds)"
-image=$("$retry" guix system -L "$repo/modules" image -t iso9660 "$repo/systems/installer.scm")
+cp $image "guix-installer-$date.iso"
-cp $image "./guix-installer-$date.iso"