Update the debian package build to use the generated man pages

Summary:
The debian package build process needs to build the man pages and not
only copy them from the source since they are generated.

Test Plan:
Ran it on my personal repo
https://launchpad.net/~fabcien/+archive/ubuntu/bitcoin-abc/+packages

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7462