Add command for building local nix package

shell.nix

@@ -4,12 +4,17 @@ let
     config = { };
     overlays = [ ];
   };
+  build-command = pkgs.writeScriptBin "build-nix-package" ''
+    nix-build -E 'with import <nixpkgs> {}; callPackage ./default.nix {}'
+  '';
 in
 pkgs.mkShell {
-  packages = with pkgs; [
+  packages =
+    (with pkgs; [
       cargo
       rustc
       rustfmt
       clippy
-  ];
+    ])
+    ++ [ build-command ];
 }