Fix fetching manpages for 0.22.3

Summary:
Although this was fixed in later versions (see D7677), 0.22.3 still suffers from not being able to generate
manpages while headless. This is a temporary fix.

Test Plan:
./scripts/fetch_documenation.sh
Also ran a similar patch in CI staging.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7678