Update the AUR packages builds to use the generated man pages
The man pages are no longer part of the source tree and need to be
generated for embedding into the packages. Note that xvfb-run is used
to make sure bitcoin-qt --help can run in a headless environment.
Test Plan: Ran the update-aur.sh script in a container for both packages.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7454