fix(writableStoreOverlay): append dont clear

lib.optional returns the empty set [] when a check is false.
In this case it causes the options to be set to an empty list which the module
does not allow. So append the new options if true, and append [] when false.

Signed-off-by: Brian McGillion <bmg.avoin@gmail.com>

Author: brianmcgillion

nixos-modules/microvm/mounts.nix

@@ -87,7 +87,8 @@ lib.mkIf config.microvm.guest.enable {
           upperdir = "${writableStoreOverlay}/store";
           workdir = "${writableStoreOverlay}/work";
         };
-        options = lib.optional isRwStoreVirtiofsShare "userxattr";
+      } // lib.optionalAttrs isRwStoreVirtiofsShare {
+        options = lib.mkAfter [ "userxattr" ];
       };
     }
   ) {