MDL-72744 core_grades: Escape formulas when exporting spreadsheets

lib/csvlib.class.php

@@ -449,6 +449,9 @@ class csv_export_writer {
             }
         }
         $delimiter = csv_import_reader::get_delimiter($this->delimiter);
+        foreach ($row as $key => $value) {
+            $row[$key] = \core\dataformat::escape_spreadsheet_formula($value);
+        }
         fputcsv($this->fp, $row, $delimiter, $this->csvenclosure, '\\');
     }