convert unsafe fs::path std::string conversion calls [6/7] (wallet)

Summary:
This is a partial backport of core#22937 [2f/2g]
https://github.com/bitcoin/bitcoin/pull/22937/commits/6544ea5035268025207d2402db2f7d90fde947a6

The backport for this commit was split in to multiple commits to make review easier. This part deals with converting fs::string() calls that
will become unsafe when migrating from boost::filesystem to std::filesystem.

Depends on D10799

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10800