add MUJOCO_LIB path

[vyeevani]

for folks that are downloading mujoco via python (uv or just straight up pip), it's tricky to setup this library. would be nice to the option to override the mujoco library path, which would allow pointing directly at the shared library instead of assuming it's at the $(MUJOCO_DIR)/lib

[kanarus]

Thank you for pointing this out.

rusty_mujoco 0.2 has switched from MUJOCO_DIR to MUJOCO_LIB, which should now be the path of the directory containing libmujoco.so (Unix-like) or mujoco.lib (Windows)! This means it doesn't depend on the directory structure of MuJoCo's official GitHub Releases.

(Additionally, when mujoco is installed as a system-wide library, no environment variable is needed at all.)