fix cache dir in builder tests
CONTRIBUTOR
Nice, is that the reason one cannot rerun the tests without deleting the cache? 

2020-05-10T21:41:13Z
https://github.com/huggingface/datasets/pull/69#issuecomment-626393940
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Yes exactly. It was not using the temporary dir for tests.
2020-05-10T23:14:47Z
https://github.com/huggingface/datasets/pull/69#issuecomment-626404611
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minor fix