ModelDerivatives::Compute makes thread-unsafe call to mj_transitionFD

Here: https://github.com/google-deepmind/mujoco_mpc/blob/79db2bc0cc8e1ca4e96af0859718c265cdd22caf/mjpc/planners/model_derivatives.cc#L96

multiple threads are calling `mjd_transitionFD` on the same `mjData` instance. Unfortunately, `mjd_transitionFD` takes a **non-const** `mjData*`, and performs MuJoCo stack allocations. Those allocations are not thread safe and will get mixed up between threads, so the resulting computation is likely nonsense.

This was discovered by running tests with ThreadSanitizer.