ci: use spawn for multiprocessing in nbdev_test #1782

Job	Run time
test-m3-performance	2m 57s
efficiency-tests	2m 6s
Run tests for cp39-manylinux_x86_64	2m 52s
Run tests for cp39-manylinux_aarch64	2m 54s
Run tests for cp39-win_amd64	2m 18s
Run tests for cp39-macosx_x86_64	1s
Run tests for cp39-macosx_arm64	1s
Run tests for cp310-manylinux_x86_64	2m 56s
Run tests for cp310-manylinux_aarch64	2m 55s
Run tests for cp310-win_amd64	2m 46s
Run tests for cp310-macosx_x86_64	58s
Run tests for cp310-macosx_arm64	51s
Run tests for cp311-manylinux_x86_64	2m 56s
Run tests for cp311-manylinux_aarch64	2m 5s
Run tests for cp311-win_amd64	2m 38s
Run tests for cp311-macosx_x86_64	26s
Run tests for cp311-macosx_arm64	58s
Run tests for cp312-manylinux_x86_64	2m 6s
Run tests for cp312-manylinux_aarch64	2m 54s
Run tests for cp312-win_amd64	2m 40s
Run tests for cp312-macosx_x86_64	1s
Run tests for cp312-macosx_arm64	12s
publish-nightly-wheels	1s
	40m 32s

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