In order to copy the resulting .xcarchive file, you will need to add a post-build trigger to your Bot. Once a build is complete and the archive is successful, you have access to the XCS_ARCHIVE environment variable. This will have a path, something similar to:
/Library/Developer/XcodeServer/Integrations/Integration-5fa0beca97c1a3e60628eee97e10a69b/Archive.xcarchive
Use that path to copy the archive to your desired destination. Please note, many of the Xcode Server directories have limited access. You many not be able to access those directories with your current user account, and even sudo-ing may not work.
Additional information about the Xcode Server environment variables can be found here: http://faq.sealedabstract.com/xcodeCI/