[BUG] Various Groupby JIT tests fail when run alone

Locally, groupby JIT tests seem to fail with a mismatched dtype, but not if the whole test suite is run.

```
FAILED python/cudf/cudf/tests/groupby/test_apply.py::test_groupby_apply_jit_unary_reductions[small-min-int32] - AssertionError: Attributes of Series are different
```

This is probably due to an improper cache hit on the kernel somewhere. 