upgrade: fix execution with new enter behavior

Signed-off-by: Luca Di Maio <luca.dimaio1@gmail.com>

distrobox-upgrade

@@ -255,6 +255,6 @@ for container in ${container_name}; do
 	printf >&2 "\033[1;31m Upgrading %s...\n\033[0m" "${container}"
 	# shellcheck disable=SC2086,SC2248
 	"${distrobox_path}"/distrobox-enter \
-		${distrobox_flags} ${container} -- \
+		${distrobox_flags} ${container} -- sh -c \
 		"command -v su-exec 2>/dev/null && su-exec root /usr/bin/entrypoint --upgrade || sudo -S /usr/bin/entrypoint --upgrade"
 done