pkgs/tools/misc/file: add cannot-use-fetchpatch warning

As requested here: https://github.com/NixOS/nixpkgs/pull/168413#pullrequestreview-988550409

pkgs/tools/misc/file/default.nix

@@ -1,5 +1,10 @@
 { lib, stdenv, fetchurl, file, zlib, libgnurx }:
 
+# Note: this package is used for bootstrapping fetchurl, and thus
+# cannot use fetchpatch! All mutable patches (generated by GitHub or
+# cgit) that are needed here should be included directly in Nixpkgs as
+# files.
+
 stdenv.mkDerivation rec {
   pname = "file";
   version = "5.41";