You need to be Confluence Administrator to perform the following steps.
The download link for the SharePoint client-side solution package is provided in Confluence since Confluence acts as provider for the SharePoint client-side solution. The package is generated on the fly when you click the download link. You will get an adjusted package which matches your Confluence environment.
Therefore you cannot use a solution package generated on a specific Confluence environment for another Confluence environment (for example staging and production environment).
The download link for the client-side solution package is provided within the Confluence Administration under Administration > Configure SharePoint Online > SharePoint Solution Package Download.
Image 1: Download link (click to enlarge)
Click on the download link. The following popup will be displayed: