Add CI builds on Ubuntu 20 with GCC 9, GCC 10, Clang 10, and Clang 11.
Add CI builds on Ubuntu 20 with GCC 9, GCC 10, Clang 10, and Clang 11.

PiperOrigin-RevId: 461500095
Change-Id: I812d1ac2a864e404c3052b3aa65443c5eb975ccf

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