lib: set return value to exit status code
lib: set return value to exit status code

The expect plugin docs contains the following description:

> ## configuration parameters
>
> * action: using `exit` makes Fluent Bit abort with status code `255`

We need to keep the return value for use in the status code.

Signed-off-by: Takumi Shotoku <5937a31c8b958467faf63a09d8c1f70b5df15641@gmail.com>

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