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