feat(check-diff): POSIX compliance

Signed-off-by: Francesco Noacco <francesco.noacco@secomind.com>

Author: noaccOS

scripts/check-diff.sh

@@ -1,13 +1,12 @@
-#! /usr/bin/env bash
+#!/bin/sh
 
 # Copyright 2023 SECO Mind Srl
 # SPDX-License-Identifier: Apache-2.0
 
-set -exEuo pipefail
+set -exu
 
-DIFF=$(git diff --exit-code)
-
-if [[ "$?" != 0 ]] ; then
+if ! DIFF=$(git diff --exit-code)
+then
     echo "differences between the generated files" >&2
     echo "$DIFF"
     exit 1