init: Use existing chainman
Summary:
This is a backport of core#21866 [1/12]
https://github.com/bitcoin/bitcoin/pull/21866/commits/464c313e304cef04a82e14f736e3c44ed5604a4e
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D11646