vmtests: use root-images:20240415.162748 in fetch-data.sh

Previously the fetch-data was defaulting to root-images:latest that is
more than a year old now (!) but most importantly, not multi-arch as
this new tag.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

tests/vmtests/fetch-data.sh

@@ -4,7 +4,7 @@ set -eu -o pipefail
 
 
 OCIORG=quay.io/lvh-images
-ROOTIMG=$OCIORG/root-images
+ROOTIMG=$OCIORG/root-images:20240415.162748@sha256:2637beacabbb48e2ee89a8f296a123142257ae10616308f81e7210ac85b92789
 KERNIMG=$OCIORG/kernel-images
 CONTAINER_ENGINE=${CONTAINER_ENGINE:-docker}
 KERNEL_VERS="$@"