macOS CI often fails with SIGABRT

[RalfJung]

For instance:

• https://github.com/rust-lang/miri/actions/runs/7368440840
• https://github.com/rust-lang/miri/actions/runs/7374983002
• https://github.com/rust-lang/miri/actions/runs/7382324074
• https://github.com/rust-lang/miri/actions/runs/7405632573

Also see this Zulip discussion.

[RalfJung]

Looks like it might have to do with using the default system allocator instead of jemalloc: rust-lang/rust#92173.