fix: Set overlay options when needed

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

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 = [ "userxattr" ];
       };
     }
   ) {