Format .nix files

... with nixfmt (rfc style)

tests/functional/lang/eval-okay-xml.nix

@@ -10,12 +10,31 @@ rec {
 
   c = "foo" + "bar";
 
-  f = {z, x, y}: if y then x else z;
+  f =
+    {
+      z,
+      x,
+      y,
+    }:
+    if y then x else z;
 
   id = x: x;
 
-  at = args@{x, y, z}: x;
+  at =
+    args@{
+      x,
+      y,
+      z,
+    }:
+    x;
 
-  ellipsis = {x, y, z, ...}: x;
+  ellipsis =
+    {
+      x,
+      y,
+      z,
+      ...
+    }:
+    x;
 
 }