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-tojson.nix

@@ -1,13 +1,26 @@
-builtins.toJSON
-  { a = 123;
-    b = -456;
-    c = "foo";
-    d = "foo\n\"bar\"";
-    e = true;
-    f = false;
-    g = [ 1 2 3 ];
-    h = [ "a" [ "b" { "foo\nbar" = {}; } ] ];
-    i = 1 + 2;
-    j = 1.44;
-    k = { __toString = self: self.a; a = "foo"; };
-  }
+builtins.toJSON {
+  a = 123;
+  b = -456;
+  c = "foo";
+  d = "foo\n\"bar\"";
+  e = true;
+  f = false;
+  g = [
+    1
+    2
+    3
+  ];
+  h = [
+    "a"
+    [
+      "b"
+      { "foo\nbar" = { }; }
+    ]
+  ];
+  i = 1 + 2;
+  j = 1.44;
+  k = {
+    __toString = self: self.a;
+    a = "foo";
+  };
+}