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/derivation/advanced-attributes-defaults.nix

@@ -2,5 +2,8 @@ derivation {
   name = "advanced-attributes-defaults";
   system = "my-system";
   builder = "/bin/bash";
-  args = [ "-c" "echo hello > $out" ];
+  args = [
+    "-c"
+    "echo hello > $out"
+  ];
 }