Fix ANN bench ground truth generation for k>1024 #4075

Job	Run time
devcontainer / build (amd64, 12.0, conda)	43m 17s
checks / check-style	1m 31s
checks / other-checks	39s
devcontainer / build (amd64, 12.0, pip)	45m 38s
wheel-build-pylibraft	0s
conda-cpp-build	0s
conda-cpp-checks	0s
conda-python-build	0s
conda-cpp-tests	0s
wheel-tests-pylibraft	0s
conda-python-tests	0s
docs-build	0s
wheel-build-raft-dask	0s
wheel-tests-raft-dask	0s
pr-builder	0s
	1h 31m 5s

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