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-okay-flatten.nix

@@ -2,7 +2,19 @@ with import ./lib.nix;
 
 let {
 
-  l = ["1" "2" ["3" ["4"] ["5" "6"]] "7"];
+  l = [
+    "1"
+    "2"
+    [
+      "3"
+      [ "4" ]
+      [
+        "5"
+        "6"
+      ]
+    ]
+    "7"
+  ];
 
   body = concat (flatten l);
 }