fix(sddm): add theme to system packages so the path will be symlinked

modules/nixos/sddm.nix

@@ -17,4 +17,9 @@
       sddm-astronaut
     ];
   };
+
+  # Make the theme available in the system path so sddm can find it
+  environment.systemPackages = with pkgs; [
+    sddm-astronaut
+  ];
 }