Add ruff.toml to exclude .xsh files from ruff linting

Ruff cannot parse xonsh syntax ($(), @()), so xonsh scripts
must be excluded via force-exclude.

flake.nix

@@ -133,7 +133,7 @@
               {
                 "s390x-linux" = s390xOverlays;
               }
-              .${target} or [];
+              .${target} or [ ];
             pkgs = import nixpkgs {
               system = "x86_64-linux";
               crossSystem.config =
@@ -152,13 +152,16 @@
                   inherit (pkgs) attic-client;
                 };
               }
-              .${target} or {};
+              .${target} or { };
           in
           {
             name = target;
-            value = packages // crossOnlyPackages // {
-              default = packages.hello-world;
-            };
+            value =
+              packages
+              // crossOnlyPackages
+              // {
+                default = packages.hello-world;
+              };
           }
         ) (builtins.attrNames crossTargets)
       );

ruff.toml

@@ -0,0 +1,2 @@
+force-exclude = true
+extend-exclude = ["*.xsh"]