Make m_cached_blocks_path mutable. Make ArgsManager::GetBlocksDirPath() const.

Summary:
This is a backport of core#21850 [1/6]
https://github.com/bitcoin/bitcoin/pull/21850/commits/716de29dd8672d28bfe0a08424e9958fe61054d2

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11952