Script filename from SasUri cannot contain /

I have a script source SasUri like:

```
https://xxx.blob.windows.net/container/foo/bar.sh
```

According to:

https://github.com/Azure/run-command-handler-linux/blob/6efb77e0fec9b1ff87c980d1a207f0fc73493b8a/pkg/download/blob.go#L82-L99

`fileName` becomes `foo/bar.sh`. However, directory `foo` doesn't exist so the `open` below always fails.

Suggest using fixed file name like a GUID. Otherwise you have to deal with `..` and `/` in paths which could cause a lot of security troubles.

從 pkg/download/blob.go 中 issue 所引用的檔名擷取和檔案開啟程式碼開始。追蹤 SasUri 路徑如何變成 fileName，然後驗證對 foo/bar.sh 以及包含 .. 或 / 的路徑的處理。完成的標準是：包含巢狀路徑元件的腳本 URI 可以在不出現缺少目錄的失敗或不安全路徑行為的情況下被處理。

由索引模型根據 Issue 內容生成。