validation: Pass in chain to FindBlockPos+SaveBlockToDisk

Summary:
This is a backport of core#21055 [13/16]
https://github.com/bitcoin/bitcoin/pull/21055/commits/fee73347c0015e4c24595c9708831d76cd6eec8c

Depends on D11219

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11220