Escape dollar sign for saving content into crontab

The crontab contents can contain dollar sign (e.g. $(date)).
The current code does not escape it which transform it to actual values.
In this commit a replacement to escape dollar sign is added to the previous replacements.

Author: Erfans

lib/internal/Magento/Framework/Crontab/CrontabManager.php

@@ -203,7 +203,7 @@ private function getCrontabContent()
      */
     private function save($content)
     {
-        $content = str_replace(['%', '"'], ['%%', '\"'], $content);
+        $content = str_replace(['%', '"', '$'], ['%%', '\"', '\$'], $content);
 
         try {
             $this->shell->execute('echo "' . $content . '" | crontab -');