Remove stringify call from Yaml serializer (chapel-lang#22833)

Fix a test failure following
chapel-lang#22068 by replacing a call to
`stringify` with `"%?".format()` in the Yaml module.

- [x] yaml tests passing

[ reviewed by @mppf ] - Thanks!

modules/packages/YAML.chpl

@@ -1753,7 +1753,7 @@ module YAML {
       } else if (isClassType(t) && val == nil) || isNothingType(t) {
         return b"~";
       } else {
-        return stringify(val);
+        return try! "%?".format(val);
       }
     }