Format .nix files

This does not include any automation for the release branch, but
is based on the configuration of https://github.com/NixOS/nix/pull/12349

    pre-commit run -a nixfmt-rfc-style

tests/functional/lang/eval-fail-missing-arg.nix

@@ -1 +1,12 @@
-({x, y, z}: x + y + z) {x = "foo"; z = "bar";}
+(
+  {
+    x,
+    y,
+    z,
+  }:
+  x + y + z
+)
+  {
+    x = "foo";
+    z = "bar";
+  }