$dynamicRef doesn't find $dynamicAnchor overrides in root schemas without $id

When a root schema has no $id and uses $ref to pull in a template containing $dynamicRef, the $dynamicRef ignores any $dynamicAnchor overrides defined in the root schema's $defs. It falls back to the template's own $dynamicAnchor instead.

Filed from python-jsonschema/jsonschema#1497. The root cause is in referencing — see that issue for the minimal reproducer.