nixos mkDerivation godot mono version (C# support) - godot

Question

i'm on nixos 20.09 and try to build godot-3.2.3-stable with support for C#.
I'm following the instruction on the godot docs, which states that one first has to enable the mono module and then generate the mono-glue. I tried to adapt the default.nix of the NixOS/nixpkgs repository to my needs but failed.
What i tried is to first generate the glue like mentioned in the godot docs and then compile with C# support.
with import <nixpkgs> {};
let options = {
pulseaudio = true;
};`
in
stdenv.mkDerivation rec {
name = "tools_";
version = "3.2.3";
src = ./godot-3.2.3-stable.tar.gz;
nativeBuildInputs = with pkgs; [ pkg-config ];
buildInputs = with pkgs; [ scons mono6 msbuild dotnetPackages.Nuget xorg.libX11 xorg.libXcursor
xorg.libXinerama xorg.libXrandr xorg.libXrender xorg.libXi xorg.libXext
xorg.libXfixes freetype openssl alsaLib libpulseaudio libGLU zlib yasm
];
patches = [
./pkg_config_additions.patch
./dont_clobber_environment.patch
];
enableParallelBuilding = true;
sconsFlags = "target=release_debug platform=x11 tools=yes module_mono_enabled=yes mono_glue=no";
preConfigure = ''
sconsFlags+=" ${lib.concatStringsSep " " (lib.mapAttrsToList (k: v: "${k}=${builtins.toJSON v}") options)}"
'';
outputs = [ "out" ];
installPhase = ''
mkdir -p "$out/bin"
cp bin/godot.* $out/bin/godot
'';
}
But if i run this with nix-build i get the error:
Checking for `thread_local` support... supported
Mono root directory not found. Using pkg-config instead
/nix/store/vnyfysaya7sblgdyvqjkrjbrb0cy11jf-bash-4.4-p23/bin/sh: pkg-config: command not found
OSError: 'pkg-config monosgen-2 --libs-only-L' exited 127:
File "/build/godot-3.2.3-stable/SConstruct", line 617:
SConscript("modules/SCsub")
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 661:
return method(*args, **kw)
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 598:
return _SConscript(self.fs, *files, **subst_kw)
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 287:
exec(compile(scriptdata, scriptname, 'exec'), call_stack[-1].globals)
File "/build/godot-3.2.3-stable/modules/SCsub", line 21:
SConscript(name + "/SCsub") # Built-in.
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 661:
return method(*args, **kw)
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 598:
return _SConscript(self.fs, *files, **subst_kw)
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Script/SConscript.py", line 287:
exec(compile(scriptdata, scriptname, 'exec'), call_stack[-1].globals)
File "/build/godot-3.2.3-stable/modules/mono/SCsub", line 34:
mono_configure.configure(env, env_mono)
File "/build/godot-3.2.3-stable/modules/mono/build_scripts/mono_configure.py", line 351:
tmpenv.ParseConfig("pkg-config monosgen-2 --libs-only-L")
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Environment.py", line 1612:
return function(self, self.backtick(command))
File "/nix/store/ypb1lsl610mw3s76p232hjmnhwakjb06-scons-4.0.1/lib/python3.8/site-packages/SCons/Environment.py", line 599:
raise OSError("'%s' exited %d" % (command, status))
builder for '/nix/store/acpp99hr7q92a7j1bk2nga00n4v8za7z-tools_.drv' failed with exit code 2
error: build of '/nix/store/acpp99hr7q92a7j1bk2nga00n4v8za7z-tools_.drv' failed
I'm sorry but i'm totally lost there, since this is my first derivation i am trying to build. I have already searched the nixpkgs manual but couldn't solve my problem. Probably due to the fact that i'm not exactly sure what i am looking for. Also i was asking myself if this is going to work anyways since the godot docs state that after generating the glue one has to:
### Build binaries normally
# Editor
scons p=x11 target=release_debug tools=yes module_mono_enabled=yes
and i don't know if i could then simply rerun the above mentioned derivation with the appropriate sconsFlags or if and how i could simply write one file which does the same job done.
If someone could help me and point me in the right direction so that i at least would know what i exactly have to search for i would very much appreciate it. Thanks in advance.