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.

Start in pkg/download/blob.go at the filename extraction and file opening code referenced in the issue. Trace how the SasUri path becomes fileName, then verify handling for foo/bar.sh and paths containing .. or /. Done means a script URI with nested path components can be processed without the missing-directory failure or unsafe path behavior.

Written by the indexing model from the issue text.