nixos/fontconfig: fix 50-user.conf handling

Apparently, edf2541f02 was missed while
rebasing https://github.com/NixOS/nixpkgs/pull/93562.

Provide 50-user.conf in fontconfig if includeUserConf is true (the
default), and don't try removing the non-existent one if it's disabled

Fixes https://github.com/NixOS/nixpkgs/issues/95685
Fixes https://github.com/NixOS/nixpkgs/issues/95712

nixos/modules/config/fonts/fontconfig.nix

@@ -204,8 +204,10 @@ let
     ln -s ${renderConf}       $dst/10-nixos-rendering.conf
 
     # 50-user.conf
-    ${optionalString (!cfg.includeUserConf) ''
-    rm $dst/50-user.conf
+    # Since latest fontconfig looks for default files inside the package,
+    # we had to move this one elsewhere to be able to exclude it here.
+    ${optionalString cfg.includeUserConf ''
+    ln -s ${pkg.out}/etc/fonts/conf.d.bak/50-user.conf $dst/50-user.conf
     ''}
 
     # local.conf (indirect priority 51)