Serverless export env version 2.0

The API has changed for this package and we now need to add `--all` to the export-env command to achieve the same as before.

More info here - https://www.npmjs.com/package/serverless-export-env

Looks like we also need to add:
```
export-env:
    overwrite: true
```
to the custom section of serverless.yml

The Outputs section is also not being written to the env file anymore