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`.
{
    auto get_net = [&](const std::string& arg) {
        LOCK(cs_args);
        util::SettingsValue value = util::GetSetting(m_settings,  "", SettingName(arg),
             false,
             true);
        return value.isNull() ? false : value.isBool() ? value.get_bool() : InterpretBool(value.get_str());
    };
    const bool fRegTest = get_net("-regtest");
    const bool fSigNet  = get_net("-signet");
    const bool fTestNet = get_net("-testnet");
    const bool is_chain_arg_set = IsArgSet("-chain");
    if ((int)is_chain_arg_set + (int)fRegTest + (int)fSigNet + (int)fTestNet > 1) {
        throw std::runtime_error("Invalid combination of -regtest, -signet, -testnet and -chain. Can use at most one.");
    }
    if (fRegTest)
        return CBaseChainParams::REGTEST;
    if (fSigNet) {
        return CBaseChainParams::SIGNET;
    }
    if (fTestNet)
        return CBaseChainParams::TESTNET;
    return GetArg("-chain", CBaseChainParams::MAIN);
}
bool ArgsManager::UseDefaultSection(const std::string& arg) const