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`.
{
    {
        LOCK(cs_args);
        m_settings.ro_config.clear();
        m_config_sections.clear();
    }
    const std::string confPath = GetArg("-conf", BITCOIN_CONF_FILENAME);
    fsbridge::ifstream stream(GetConfigFile(confPath));
    
    if (IsArgSet("-conf") && !stream.good()) {
        error = strprintf("specified config file \"%s\" could not be opened.", confPath);
        return false;
    }
    
    if (stream.good()) {
        if (!ReadConfigStream(stream, confPath, error, ignore_invalid_keys)) {
            return false;
        }
        
        
        bool use_conf_file{true};
        {
            LOCK(cs_args);
            if (auto* includes = util::FindKey(m_settings.command_line_options, "includeconf")) {