feat: add formatter for Nix files

flake.nix

@@ -13,8 +13,15 @@
     };
   };
 
-  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
-    flake-utils.lib.eachSystem ["x86_64-linux"] (system:
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      rust-overlay,
+    }:
+    flake-utils.lib.eachSystem [ "x86_64-linux" ] (
+      system:
       let
         overlays = [ (import rust-overlay) ];
         pkgs = import nixpkgs { inherit system overlays; };
@@ -31,20 +38,24 @@
           cargoLock.lockFile = ./Cargo.lock;
           doCheck = true;
         };
-      in {
-        packages.rustPackage = appRustBuild;
+      in
+      {
+        packages.pumoSystemInfo = appRustBuild;
+        formatter = nixpkgs.legacyPackages.${system}.nixfmt-rfc-style;
         defaultPackage = appRustBuild;
-        devShell = with pkgs; mkShell {
-          buildInputs = [
-            bacon
-            cargo
-            cargo-deny
-            cargo-msrv
-            cargo-tarpaulin
-            just
-            rustVersion
-          ];
-        };
+        devShell =
+          with pkgs;
+          mkShell {
+            buildInputs = [
+              bacon
+              cargo
+              cargo-deny
+              cargo-msrv
+              cargo-tarpaulin
+              just
+              rustVersion
+            ];
+          };
       }
     );
 }