Move to Dask nightlies in CI

[jakirkham]

Currently we have code here that is installing Dask & Distributed from source. This was needed as there were not nightlies previously

ucx-py/ci/gpu/build.sh

Lines 49 to 53 in 4c154c0

	# Install the main version of dask and distributed
	gpuci_logger "pip install git+https://github.com/dask/distributed.git@main --upgrade --no-deps"
	pip install "git+https://github.com/dask/distributed.git@main" --upgrade --no-deps
	gpuci_logger "pip install git+https://github.com/dask/dask.git@main --upgrade --no-deps"
	pip install "git+https://github.com/dask/dask.git@main" --upgrade --no-deps

However Dask & Distributed now provide nightlies ( dask/community#76 ), which can be used instead.

Changes have been made to cuDF ( rapidsai/cudf#10182 ) and Dask-CUDA ( rapidsai/dask-cuda#897 ) to use these nightlies.

Would be good if we could use these in cuML as well.

[pentschev]

Addressing this in #875