fastMNN causing timeout in `hca-bone-marrow.Rmd` when testing with `openblas-openmp`

[PeteHaitch]

From #bioc-builds > Books now being built on macOS in devel? @ 💬

The timeout happens in OSCA.workflows in the hca-bone-marrow.Rmd chapter in the integration chunk when batchelor::fastMNN() is called. This call spans five R processes that use 14.1g each and the load falls to < 1. So it looks like batchelor::fastMNN() gets stuck, but it's stuck doing what? It doesn't seem to be I/O. This only happens with openblas-openmp. I want to investigate this more. Note that this indirectly causes the timeouts you see for the other sub-books (OSCA.basic, OSCA.multisample and *OSCA.advanced) because for all of them rebook::preCompileBook() gets stuck waiting for OSCA.workflows to compute and cache the stuff that they need in order to proceed.

OSCA.workflows/inst/book/hca-bone-marrow.Rmd

Lines 111 to 122 in 9bb4528

	```{r integration}
	library(batchelor)
	library(BiocNeighbors)
	set.seed(1010001)
	merged.bone <- fastMNN(sce.bone, batch = sce.bone$Donor, subset.row = top.bone,
	BSPARAM=BiocSingular::RandomParam(deferred = TRUE),
	BNPARAM=AnnoyParam(),
	BPPARAM=bpp)
	reducedDim(sce.bone, 'MNN') <- reducedDim(merged.bone, 'corrected')
	```

[PeteHaitch]

This seemed to resolve itself, but revealed a difference in fastMNN() results across systems (#7).