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

@@ -156,7 +156,10 @@
           in
           {
             name = target;
-            value = packages // crossOnlyPackages // {
+            value =
+              packages
+              // crossOnlyPackages
+              // {
                 default = packages.hello-world;
               };
           }

ruff.toml

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