Detect the correct nvidia lib path for modern ubuntu systems (#646)

To enable GPU extensions, previously either /usr/local/nvidia/lib64
(for dockers) or /usr/lib/nvidia-\d\d\d needed to exist, but in
most of the modern debian and ubuntu systems, the nvidia libraries
are actually installed under the path /usr/lib/nvidia.

The commit corrects the path to nvidia library path so that
GPU extensions can be installed without any hacky workarounds.

mujoco_py/builder.py

@@ -31,6 +31,11 @@ def get_nvidia_lib_dir():
     docker_path = '/usr/local/nvidia/lib64'
     if exists(docker_path):
         return docker_path
+
+    nvidia_path = '/usr/lib/nvidia'
+    if exists(nvidia_path):
+        return nvidia_path
+
     paths = glob.glob('/usr/lib/nvidia-[0-9][0-9][0-9]')
     paths = sorted(paths)
     if len(paths) == 0: