[gitian-build] Move manifest files to output directory
This puts the manifest files where we expect them when doing releases.
Depends on D4231
./gitian-build.py -n -c -b gitianuser v0.20.3
Verify manifest files are included with binaries.
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4232