nix: init at 2.25.0

[puffnfresh]

I haven't done a build in a chroot so hopefully I got all the dependencies.

The output executable is not useful at the moment. If you try to run this package, it will crash, because the Nix codebase currently has lots of problems with handling Windows paths. Even after the big task of fixing paths, we'll have to handle process management, making a useful Windows standard environment, adding tests, and lots of other things.

I think it makes sense to not merge this package, until Nix has a minimal working Windows release. What do you think @Ericson2314?

Fixes #22459.

[Ericson2314]

Yeah fine with me just keeping this draft for now.

[ognevny]

CLANG64 fails with:

FAILED: src/libutil/libnixutil.dll.p/windows_environment-variables.cc.obj 
  "clang++" "-Isrc/libutil/libnixutil.dll.p" "-Isrc/libutil" "-I../nix-2.25.0/src/libutil" "-Isrc/libutil/windows" "-I../nix-2.25.0/src/libutil/windows" "-fdiagnostics-color=always" "-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST" "-D_FILE_OFFSET_BITS=64" "-Wall" "-Winvalid-pch" "-std=c++2a" "-include" "config-util.hh" "-Wdeprecated-copy" "-Werror=suggest-override" "-Werror=switch" "-Werror=switch-enum" "-Werror=unused-result" "-Wignored-qualifiers" "-Wimplicit-fallthrough" "-Wno-deprecated-declarations" "-D_WIN32_WINNT=0x0602" "-march=nocona" "-msahf" "-mtune=generic" "-O2" "-pipe" "-Wp,-D_FORTIFY_SOURCE=2" "-fstack-protector-strong" "-Wp,-D__USE_MINGW_ANSI_STDIO=1" "-DBOOST_CONTEXT_DYN_LINK=1" "-DBOOST_COROUTINES_DYN_LINK=1" "-DBOOST_ALL_NO_LIB" "-pthread" -MD -MQ src/libutil/libnixutil.dll.p/windows_environment-variables.cc.obj -MF "src/libutil/libnixutil.dll.p/windows_environment-variables.cc.obj.d" -o src/libutil/libnixutil.dll.p/windows_environment-variables.cc.obj "-c" ../nix-2.25.0/src/libutil/windows/environment-variables.cc
  ../nix-2.25.0/src/libutil/windows/environment-variables.cc:16:31: error: non-constant-expression cannot be narrowed from type 'DWORD' (aka 'unsigned long') to 'wchar_t' in initializer list [-Wc++11-narrowing]
     16 |     std::wstring value{L'\0', bufferSize};
        |                               ^~~~~~~~~~
  ../nix-2.25.0/src/libutil/windows/environment-variables.cc:16:31: note: insert an explicit cast to silence this issue
  
     16 |     std::wstring value{L'\0', bufferSize};
  
        |                               ^~~~~~~~~~
  
        |                               static_cast<wchar_t>( )
  
  1 error generated.

[Ericson2314]

NixOS/nix#11856 OK this would fix the one build failure.