Build the debian package for both chains

Summary:
This adds a --network option to the debian-packages.sh script to
allow building package for the BCHN chain.

Depends on D8289.

Test Plan:

./contrib/release/debian-packages.sh -d -n BCHN fabien

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8295