Add buildah sign task as image

Signed-off-by: Enrique Belarte Luque <[email protected]>

.tekton/buildah-oci-ta.yaml

@@ -536,7 +536,7 @@ spec:
           echo "Mounting $IMAGE"
         )
         sign_cmd=$(printf "%q" "${sign_cmd_array[@]}")
-        buildah_cmd="$buildah_cmd && buildah mount \"$IMAGE\" && echo \"Mounting\""
+        buildah_cmd="$buildah_cmd && buildah from \"$IMAGE\" && echo \"Mounting $IMAGE\""
 
         if [ "${HERMETIC}" == "true" ]; then
           # enabling loopback adapter enables Bazel builds to work in hermetic mode.