ci(shellcheck): fix exit code propagation (#64)

Return properly the exit code status of shellcheck command when looping
through shell script files.

run-tests.sh

@@ -28,7 +28,7 @@ check_commitlint () {
 }
 
 check_shellcheck () {
-    find . -name "*.sh" -exec shellcheck {} \;
+    find . -name "*.sh" -exec shellcheck {} \+
 }
 
 check_pydocstyle () {

start.sh

@@ -1,7 +1,7 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 # This file is part of REANA.
-# Copyright (C) 2017, 2018, 2021, 2023 CERN.
+# Copyright (C) 2017, 2018, 2021, 2023, 2024 CERN.
 #
 # REANA is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.