Update the AUR packages builds to use the generated man pages

Summary:
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