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-search-path.nix

@@ -6,5 +6,16 @@ assert isFunction (import <nix/fetchurl.nix>);
 assert length __nixPath == 5;
 assert length (filter (x: baseNameOf x.path == "dir4") __nixPath) == 1;
 
-import <a.nix> + import <b.nix> + import <c.nix> + import <dir5/c.nix>
-  + (let __nixPath = [ { path = ./dir2; } { path = ./dir1; } ]; in import <a.nix>)
+import <a.nix>
++ import <b.nix>
++ import <c.nix>
++ import <dir5/c.nix>
++ (
+  let
+    __nixPath = [
+      { path = ./dir2; }
+      { path = ./dir1; }
+    ];
+  in
+  import <a.nix>
+)