fix: Remove incorrect ./parts/fleet-data.nix import from flake.nix and use correct flake-parts structure

parts/packages.nix

@@ -0,0 +1,27 @@
+# Build artifacts (ISOs, LXC containers, etc.)
+{
+  inputs,
+  self,
+  lib,
+  config,
+  ...
+}:
+{
+  perSystem =
+    { system, ... }:
+    lib.mkIf (system == "x86_64-linux") {
+      packages =
+        let
+          fleet = self.lib.mkFleet { inherit inputs lib config; };
+          artifacts = import ../installer/artifacts.nix {
+            inherit
+              inputs
+              fleet
+              self
+              system
+              ;
+          };
+        in
+        artifacts;
+    };
+}