fix: Cap modin object store memory on macOS

[alexander-beedie]

We need to cap this on macOS to avoid the following error when larger values are set:

MODIN_MEMORY=8000000000 → ValueError: 
  The configured object store size (7.451GiB)
  exceeds the optimal size on Mac (2.0GiB) 

While we could also set RAY_ENABLE_MAC_LARGE_OBJECT_STORE=1, this isn't really recommended as performance degrades substantially...

(ref: ray._private.ray_constants.MAC_DEGRADED_PERF_MMAP_SIZE_LIMIT)

[alexander-beedie]

Would be nice to get #186 merged too, so macOS dev/bench goes smoothly :)