[WEBSITE] Update the link to the new PPA for the BCHN network variant

Summary:
It is not possible to have the same file names for different packages as
part of the same PPA, so we need to use another one.

Test Plan:

make serve

Check the link.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8311