Add missing dependency for install-manpages-html

Summary:
This build has been broken for a week but went unnoticed until now
due to cached files not triggering the build in the first place (during web deployments).

This patch adds the missing dependency, as well as ensuring the manpages can be generated headlessly
(this is essentially re-applying D5361 after D7446 which did not preserve this behavior)

Test Plan: @bot build-docs passes: https://build.bitcoinabc.org/viewLog.html?buildId=73494&buildTypeId=BitcoinABC_BitcoinAbcStaging&tab=buildResultsDiv

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7677