Don't run dask tests twice when testing wheels

[benfred]

The wheel unittest CI was running the dask tests twice in GHA. Fix by adjusting the pytest --ignore param to work like pytest expects.

[benfred]

You can see the dask tests running twice in logs like https://github.com/rapidsai/cuml/actions/runs/4412150319/jobs/7731536409 -

with

2023-03-14T05:07:59.9552965Z [gw1] [ 94%] PASSED python/cuml/tests/dask/test_dask_nearest_neighbors.py::test_compare_skl[10-False-1-10-5-10-300] 

being reported by the first run (thats using pytest-xdist) and

2023-03-14T05:56:05.5596040Z python/cuml/tests/dask/test_dask_nearest_neighbors.py::test_compare_skl[10-False-1-10-5-10-300] PASSED [ 86%]

being reported by the second pass thats only running the dask tests

With this change - this test should only be run once.

[cjnolet]

/merge