chore: add package build to flake

flake.nix

@@ -33,5 +33,44 @@
           };
         }
       );
+
+      packages = forAllSystems (
+        { pkgs }:
+        {
+          fmm = pkgs.stdenv.mkDerivation {
+            pname = "fmm";
+            version = "0.1.0";
+            src = ./.;
+
+            strictDeps = true;
+
+            nativeBuildInputs = with pkgs; [
+              godot
+            ];
+
+            buildPhase = ''
+              runHook preBuild
+
+              export HOME=$(mktemp -d)
+              mkdir -p $HOME/.local/share/godot/
+              ln -s ${pkgs.godot.export-template}/share/godot/export_templates "$HOME/.local/share/godot/"
+              mkdir -p game/build
+              cd game
+
+              echo "godot --headless --export-debug Linux ./build/fmm"
+              godot --headless --export-release Linux ./build/fmm
+
+              runHook postBuild
+            '';
+            installPhase = ''
+              find .
+              install -D -m 755 -t $out/libexec ./build/fmm
+              install -D -m 644 -t $out/libexec ./build/fmm.pck
+              install -d -m 755 $out/bin
+              ln -s $out/libexec/fmm $out/bin/fmm
+            '';
+          };
+        }
+      );
     };
 }