Make sanity check in `clustering.Rmd` more robust

[PeteHaitch]

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

stopifnot(identical(unname(which.max(by.clust.m)), memory)) in OSCA.basic/clustering.Rmd fails on lconway and kjohnson3. These might be to the difference in key cluster labels that you mentioned.

I've seen this before when testing OSCA on my M1 Mac.
Need to make this check more robust.