modules: fix fontconfig.nix to use '$dst', not 'dst'

Without the change non-default configs like:

    fonts.fontconfig.subpixel.rgba = "rgb"

fail to build the system as:

    fontconfig-conf> ln: failed to create symbolic link 'dst/': No such file or directory

nixos/modules/config/fonts/fontconfig.nix

@@ -169,7 +169,7 @@ let
   replaceDefaultConfig = defaultConfig: newConfig: ''
     rm $dst/${defaultConfig}
     ln -s ${pkg.out}/share/fontconfig/conf.avail/${newConfig} \
-          dst/
+          $dst/
   '';
 
   # fontconfig configuration package