Spoof DISPLAY on headless build servers when generating manpages for bitcoin-qt

Summary:
bitcoin-qt has a tightly coupled dependency on Qt resources being initialized before
help text is generated (see the code. it's related to translations). This is problematic for
generating the manpages because it needs to run bitcoin-qt --help. We can get around this
for now using xvfb to spoof a DISPLAY. A longer term solution would be to decouple Qt from
the help text, but a lot more thought needs to be put into if this is acceptable and if the
deviate from Core would be worth it.

Test Plan:

COMMIT_TYPE=update-manpages ./automated-commits.sh  # fails without xvfb installed

sudo apt-get install xvfb
COMMIT_TYPE=update-manpages ./automated-commits.sh  # succeeds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5361