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`.
#include <fcntl.h>
#include <sched.h>
#include <sys/resource.h>
#include <sys/stat.h>
#else
#ifdef _MSC_VER
#pragma warning(disable:4786)
#pragma warning(disable:4804)
#pragma warning(disable:4805)
#pragma warning(disable:4717)
#endif
#ifndef NOMINMAX
#define NOMINMAX
#endif
#include <codecvt>
#include <io.h> 
#include <shellapi.h>
#include <shlobj.h>
#endif
#ifdef HAVE_MALLOPT_ARENA_MAX
#include <malloc.h>
#endif
#include <boost/algorithm/string/replace.hpp>
#include <optional>
#include <thread>
#include <typeinfo>
#include <univalue.h>
const int64_t nStartupTime = GetTime();
const char * const BITCOIN_CONF_FILENAME = "bitcoin.conf";
const char * const BITCOIN_SETTINGS_FILENAME = "settings.json";