Merge #13877: utils: Make fs::path::string() always return utf-8 string on Windows

Summary:
2c3eade704 Make fs::path::string() always return utf-8 string (Chun Kuan Lee)

Pull request description:

Imbue `fs::path` with `std::codecvt_utf8_utf16` at `SetupEnvironment()`, so that default string encoding will be utf-8 inside `fs::path`.

Tree-SHA512: 0cb59464d777278decbf24771fc5ff0cb2caa7bc2fe8ee5cd36c97a2324873a3caad131f08f050393b488316ee7f4ab0b28b7fa4699e41839f8e51b9867d5118

Backport of Core PR13877
https://github.com/bitcoin/bitcoin/pull/13877/files

Test Plan:

make check
bitcoin-qt # sanity checks

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3241