[AUR] Add PKGBUILD files for the BCHN network variant

Summary:
The bitcoin-abc and bitcoin-abc-qt packages will support the ABC
network while the bitcoin-abc-bchn and bitcoin-abc-qt-bchn will
support the BCHN network.

Depends on D8289.

Test Plan:
Since the upstream repo doesn't exist yet, and the 0.22.6 release is not
ready:

ninja package_source
mkdir DUT
cp bitcoin-abc-0.22.6.tar.gz DUT/
cp -R ../contrib/aur/common/* DUT/
cp ../contrib/aur/bitcoin-abc-bchn/PKGBUILD DUT/
pushd DUT
sed -i 's/source=(.\+/source=(file:///bitcoin-abc-0.22.6.tar.gz'

PKGBUILD

echo "sha256sums=('f00')" >> PKGBUILD
updpkgsums PKGBUILD
makepkg --printsrcinfo > .SRCINFO
makepkg --clean

The build will fail at package step due to missing files during the copy
operations, this is expected.
Repeat with bitcoin-abc-qt-bchn

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8296