Add a Linux Clang build and `MUJOCO_HARDEN` to build.yml.
Add a Linux Clang build and `MUJOCO_HARDEN` to build.yml.

PiperOrigin-RevId: 450709606
Change-Id: I63525e4670d0d05ca478c0a3f8c39f15d0b2aaf3

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