image-sdk: Fix the imageroot default value

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

apertis-image-sdk.yaml

@@ -3,7 +3,7 @@
 {{ $mirror := or .mirror "https://repositories.apertis.org/apertis/" }}
 {{ $suite := or .suite "18.12" }}
 {{ $timestamp := or .timestamp "00000000.0" }}
-{{ $imageroot := or .imageroot "https://images.apertis.org/daily/$suite/$timestamp" }}
+{{ $imageroot := or .imageroot (printf "https://images.apertis.org/daily/%s/%s" $suite $timestamp) }}
 {{ $ospack := or .ospack (printf "ospack_%s-%s-%s_%s" $suite $architecture $type $timestamp) }}
 {{ $image := or .image (printf "apertis-%s-%s-%s_%s" $suite  $type $architecture $timestamp) }}