CI on MacOS runs slowly

[paciorek]

It seems to start out fine but then get bogged down in improveParamMarginals steps in test-nested-fit.R.

I suspect this is on an Intel Mac as testing on my M2 Mac runs fine (and actually much more quickly than on my Linux machine).

Not suggesting we do anything about this right now; just noting. It would be helpful to have an Intel Mac to test on and/or to test on a second Apple Silicon Mac to confirm things are fine there.

[paciorek]

On one recent run, the MacOS testing timed out after 6 hours where the Linux testing finished in 2.25 hours.