doc: use gitignoreSource

Running `make -C doc` to build the manual locally leaves .xml artifacts
in the tree. These are ignored by git, but they still get included in
the build when not using flakes, which causes the corresponding chapters
not to be built.

doc/default.nix

@@ -1,6 +1,5 @@
 { pkgs ? (import ./.. { }), nixpkgs ? { }}:
 let
-  lib = pkgs.lib;
   doc-support = import ./doc-support { inherit pkgs nixpkgs; };
 in pkgs.stdenv.mkDerivation {
   name = "nixpkgs-manual";
@@ -15,7 +14,7 @@ in pkgs.stdenv.mkDerivation {
     xmlformat
   ];
 
-  src = lib.cleanSource ./.;
+  src = pkgs.nix-gitignore.gitignoreSource [] ./.;
 
   postPatch = ''
     ln -s ${doc-support} ./doc-support/result