[SPARK-21283][CORE] FileOutputStream should be created as append mode

## What changes were proposed in this pull request?

`FileAppender` is used to write `stderr` and `stdout` files  in `ExecutorRunner`, But before writing `ErrorStream` into the the `stderr` file, the header information has been written into ,if  FileOutputStream is  not created as append mode, the  header information will be lost

## How was this patch tested?
unit test case

Author: liuxian <liu.xian3@zte.com.cn>

Closes #18507 from 10110346/wip-lx-0703.