[gitian-build] Move manifest files to output directory

Summary:
This puts the manifest files where we expect them when doing releases.

Depends on D4231

Test Plan:

./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