nixos/documentation: fix docs cross build

a few things should've used buildPackages/nativeBuildInputs to not not require
the host architecture for building docs. tested by building aarch64-linux docs
on x86_64-linux, and the result looks good.

nixos/doc/manual/default.nix

@@ -214,7 +214,7 @@ in rec {
 
   manualEpub = runCommand "nixos-manual-epub"
     { inherit sources;
-      buildInputs = [ libxml2.bin libxslt.bin zip ];
+      nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin buildPackages.zip ];
     }
     ''
       # Generate the epub manual.

nixos/modules/misc/documentation.nix

@@ -76,7 +76,7 @@ let
         } ''
           export NIX_STORE_DIR=$TMPDIR/store
           export NIX_STATE_DIR=$TMPDIR/state
-          ${pkgs.nix}/bin/nix-instantiate \
+          ${pkgs.buildPackages.nix}/bin/nix-instantiate \
             --show-trace \
             --eval --json --strict \
             --argstr libPath "$libPath" \