Do the same for ubuntu-image snap builds.

live-build/auto/build

@@ -27,9 +27,13 @@ if [ "${IMAGEFORMAT:-}" = "ubuntu-image" ]; then
 		CHANNEL="${CHANNEL:-edge}"
 		env SNAPPY_STORE_NO_CDN=1 \
 		ubuntu-image snap -c "$CHANNEL" $UBUNTU_IMAGE_ARGS \
-			-o "$PREFIX".img "$PREFIX".model-assertion
+			-O output "$PREFIX".model-assertion
+		# XXX: currently we only have one image generated, but really
+		#  we should be supporting more than one for models that
+		#  define those.
+		mv output/*.img "$PREFIX".img
 		xz -0 -T4 "$PREFIX".img
-		mv seed.manifest "$PREFIX".manifest
+		mv output/seed.manifest "$PREFIX".manifest
 	else
 		# First we need to build the gadget tree
 		make -C "config/$PREFIX-gadget"