Fix `DatasetBuilder.is_prepared()` #10499

Job	Run time
Check whether we should run tests or not	10s
Github API tests	2m 51s
Notebook tests	7m 23s
HuggingFace Python 3.10 tests	2m 41s
Core TFDS tests / Generate shards	1s
Core TFDS tests / [Python 3.10][1/4]	4m 59s
Core TFDS tests / [Python 3.11][1/4]	4m 38s
Core TFDS tests / [Python 3.12][1/4]	5m 59s
Core TFDS tests / [Python 3.10][2/4]	4m 14s
Core TFDS tests / [Python 3.11][2/4]	4m 41s
Core TFDS tests / [Python 3.12][2/4]	5m 37s
Core TFDS tests / [Python 3.10][3/4]	4m 48s
Core TFDS tests / [Python 3.11][3/4]	4m 54s
Core TFDS tests / [Python 3.12][3/4]	5m 50s
Core TFDS tests / [Python 3.10][4/4]	5m 26s
Core TFDS tests / [Python 3.11][4/4]	5m 0s
Core TFDS tests / [Python 3.12][4/4]	6m 7s
	1h 15m 19s

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