documentation-highlighter: less weird source filter

The expansion into absolute paths caused the filter to reject all the
files in some unusual circumstances (we think it's due to use of a
chroot store). This works reliably no matter where nixpkgs is located.

pkgs/misc/documentation-highlighter/default.nix

···
   };
   src = lib.sources.cleanSourceWith {
     src = ./.;
-    filter = path: type: lib.elem path (map toString [
-      ./highlight.pack.js
-      ./LICENSE
-      ./loader.js
-      ./mono-blue.css
-      ./README.md
+    filter = path: type: lib.elem (baseNameOf path) ([
+      "highlight.pack.js"
+      "LICENSE"
+      "loader.js"
+      "mono-blue.css"
+      "README.md"
     ]);
   };
 } ''