Add commands to copy the manifest files as part of the gitian building process, as they're generated in a separate location
Summary: The manifest files have been left out of releases in the past. Until this is crystallized into our process via a script, adding this reminder will be helpful.
Test Plan: Copy the files as instructed while gitian building
Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2285