chore: don't use IFD for Cargo.nix

flake.nix

@@ -27,15 +27,11 @@
         extensions = ["rust-src" "clippy" "rustfmt" "llvm-tools-preview"];
       };
 
-      generatedCargoNix = crate2nix.tools.${system}.generatedCargoNix {
-        name = "improvise";
-        src = ./.;
-      };
-
-      cargoNix = import generatedCargoNix {
-        pkgs = pkgs;
+      cargoNix = import ./Cargo.nix {
+        inherit nixpkgs pkgs;
       };
     in {
+      inherit cargoNix;
       devShells.default = pkgs.mkShell {
         nativeBuildInputs = [
           rustToolchain
@@ -55,6 +51,9 @@
         RUST_BACKTRACE = "1";
       };
 
-      packages.default = cargoNix.rootCrate.build;
+      packages = {
+        improvise = cargoNix.workspaceMembers.improvise.build;
+        default = self.packages.${system}.improvise;
+      };
     });
 }