Replace fabs with std::abs.
Replace fabs with std::abs.

Using fabs with int arguments broke the clang build on GitHub Actions, with a `-Wabsolute-value` error.

PiperOrigin-RevId: 453872474
Change-Id: I5c1278e9e628b0ea3d6f26c7ee7d6925c6276850

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco