Fix missing update to bitcoin-qt manpages

Summary:
The automated build failed to include the bitcoin-qt manpage update due to:

• Headless build agent is unable to run bitcoin-qt (not even --help)
• gen-manpages.sh not failing as expected when bitcoin-qt --help could not be run

Fixes for the above incoming. For now, we have this patch.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5359