xresources: convert font size to a string

A hack necessary because of a home-manager limitation. It works because
numbers and strings are serialized the same way for .Xresources. It
should be reverted after this issue is addressed upstream.

(cherry picked from commit aec7be4)

modules/xresources/hm.nix

@@ -7,7 +7,7 @@
   config = lib.mkIf (config.stylix.enable && config.stylix.targets.xresources.enable) {
     xresources.properties = with config.lib.stylix.colors.withHashtag; with config.stylix.fonts; {
       "*.faceName" = monospace.name;
-      "*.faceSize" = sizes.terminal;
+      "*.faceSize" = toString sizes.terminal;
       "*.renderFont" = true;
       "*foreground" = base05;
       "*background" = base00;