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-nested-list-items.nix

@@ -8,4 +8,27 @@
 #
 #        error: cannot coerce a list to a string: [ [ 1 2 3 4 5 6 7 8 ] [ 1 «4294967290 items elided» ] ]
 
-"" + (let v = [ [ 1 2 3 4 5 6 7 8 ] [1 2 3 4]]; in builtins.deepSeq v v)
+""
++ (
+  let
+    v = [
+      [
+        1
+        2
+        3
+        4
+        5
+        6
+        7
+        8
+      ]
+      [
+        1
+        2
+        3
+        4
+      ]
+    ];
+  in
+  builtins.deepSeq v v
+)