`aggregate2` method does not respect `nthread` argument if `nthread` > 1

[ChristopherEeles]

Reprex:

data(nci_TRE_small)
nci_TRE_small |>
    aggregate(assay="sensitivity", mean(viability), by=c("drug1id", "drug2id"), nthread=3)
## Uses more than three threads, as observed via `htop`.