[buildbot] Add links to bchn repo when porting code

Summary: Uses a similar syntax than core, core-gui and secp256k1 with prefix bchn.

Test Plan:

ninja check-buildbot

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13102