Reduce the number of pytest workers for Linux aarch64 CPU builds.
This may fix the apparent OOM in CI: https://github.com/jax-ml/jax/actions/runs/23090514204/job/67074614321
I ran this job locally on a t2a-48 VM and the amount of free memory was perilously low. If we limit the number of concurrent tests it should resolve the problem.
PiperOrigin-RevId: 883696377
