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`.
{
    Assert(cmd.find('=') == std::string::npos);
    Assert(cmd.at(0) != '-');
    LOCK(cs_args);
    m_accept_any_command = false; 
    std::map<std::string, Arg>& arg_map = m_available_args[OptionsCategory::COMMANDS];
    auto ret = arg_map.emplace(cmd, Arg{"", help, ArgsManager::COMMAND});
    Assert(ret.second); 
}
void ArgsManager::AddArg(const std::string& name, const std::string& help, unsigned int flags, const OptionsCategory& cat)
{
    Assert((flags & ArgsManager::COMMAND) == 0); 
    
    size_t eq_index = name.find('=');
    if (eq_index == std::string::npos) {
        eq_index = name.size();
    }
    std::string arg_name = name.substr(0, eq_index);
    LOCK(cs_args);
    std::map<std::string, Arg>& arg_map = m_available_args[cat];
    auto ret = arg_map.emplace(arg_name, Arg{name.substr(eq_index, name.size() - eq_index), help, flags});
    assert(ret.second); 
    if (flags & ArgsManager::NETWORK_ONLY) {
        m_network_only_args.emplace(arg_name);
    }
}
void ArgsManager::AddHiddenArgs(const std::vector<std::string>& names)