fontconfig: fix properly, remove NixOS module hack

This reverts 49d33d4a6b.

nixos/modules/config/fonts/fontconfig.nix

@@ -180,14 +180,8 @@ let
     mkdir -p $dst
 
     # fonts.conf
-    cp ${pkg.out}/etc/fonts/fonts.conf \
+    ln -s ${pkg.out}/etc/fonts/fonts.conf \
           $dst/../fonts.conf
-
-    # horrible sed hack to add the line that was accidentally removed
-    # from the default config in #324516
-    # FIXME: fix that, revert this
-    sed "5i <include ignore_missing="yes">/etc/fonts/conf.d</include>" -i $dst/../fonts.conf
-
     # TODO: remove this legacy symlink once people stop using packages built before #95358 was merged
     mkdir -p $out/etc/fonts/2.11
     ln -s /etc/fonts/fonts.conf \

pkgs/development/libraries/fontconfig/default.nix

@@ -69,6 +69,7 @@ stdenv.mkDerivation (finalAttrs: {
   postInstall = ''
     cd "$out/etc/fonts"
     xsltproc --stringparam fontDirectories "${dejavu_fonts.minimal}" \
+      --stringparam includes /etc/fonts/conf.d \
       --path $out/share/xml/fontconfig \
       ${./make-fonts-conf.xsl} $out/etc/fonts/fonts.conf \
       > fonts.conf.tmp