Working on "Use std::filesystem. Remove Boost Filesystem & System" #20744, I found an unfortunate [pitfall](https://github.com/bitcoin/bitcoin/pull/20744#issuecomment-946046666) -- `boost/process.hpp` depends on `boost::filesystem` when cross-compiling for Windows. It happens in Guix environment as well.

The goal of this issue is to discuss and find a solution to get rid of `boost::filesystem`.
{
    fs::path path, path_tmp;
    if (!GetSettingsPath(&path,  false) || !GetSettingsPath(&path_tmp,  true)) {
        throw std::logic_error("Attempt to write settings file when dynamic settings are disabled.");
    }
    LOCK(cs_args);
    std::vector<std::string> write_errors;
    if (!util::WriteSettings(path_tmp, m_settings.rw_settings, write_errors)) {
        SaveErrors(write_errors, errors);
        return false;
    }
    if (!RenameOver(path_tmp, path)) {
        SaveErrors({strprintf("Failed renaming settings file %s to %s\n", fs::PathToString(path_tmp), fs::PathToString(path))}, errors);
        return false;
    }
    return true;
}
bool ArgsManager::IsArgNegated(const std::string& strArg) const
{
    return GetSetting(strArg).isFalse();
}
std::string ArgsManager::GetArg(const std::string& strArg, const std::string& strDefault) const
{
    const util::SettingsValue value = GetSetting(strArg);
    return value.isNull() ? strDefault : value.isFalse() ? "0" : value.isTrue() ? "1" : value.get_str();
}
int64_t ArgsManager::GetIntArg(const std::string& strArg, int64_t nDefault) const