flake: apps: convert to 'flake-parts'

flake/apps.nix

@@ -0,0 +1,9 @@
+{ inputs, ... }:
+{
+  perSystem = { self', ... }: {
+    apps = {
+      diff-flake = inputs.futils.lib.mkApp { drv = self'.packages.diff-flake; };
+      default = self'.apps.diff-flake;
+    };
+  };
+}

flake/default.nix

@@ -21,11 +21,6 @@ let
   eachMySystem = eachSystem mySystems;
 
   systemDependant = system: {
-    apps = {
-      diff-flake = futils.lib.mkApp { drv = self.packages.${system}.diff-flake; };
-      default = self.apps.${system}.diff-flake;
-    };
-
     # Work-around for https://github.com/nix-community/home-manager/issues/3075
     legacyPackages = {
       homeConfigurations = {
@@ -68,6 +63,7 @@ flake-parts.lib.mkFlake { inherit inputs; } {
   systems = mySystems;
 
   imports = [
+    ./apps.nix
     ./checks.nix
     ./dev-shells.nix
     ./lib.nix