Detect the correct nvidia lib path for modern ubuntu systems

[wookayin]

I am glad to see that this package is still maintained (and should be), given that mujoco-py is very widely used in the research community, and we are expecting a new release for mujuco 2.1 support as #640 has been finally merged. I think this is another patch that would be very nice to include towards the next release.

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, to name a few: #583 #383 #408, where users had to do the following hack:

sudo mkdir -p /usr/lib/nvidia-000

[wookayin]

/cc @matthiasplappert @MillionIntegrals your attention on this PR would be appreciated.

[matthiasplappert]

LGTM, thanks for contributing

[wookayin]

@matthiasplappert Just to confirm: now I understand that the purpose of this detection is to ensure users to have LD_LIBRARY_PATH containing the nvidia lib directory. What shared libraries does the builder expect? Like libGL.so?

Actually, on 'a' Ubuntu machine (20.04, nvidia driver installed from official nvidia driver installation binary), the /usr/lib/nvidia directory contains only few files:

$ ls /usr/lib/nvidia
32/
egl_dummy_vendor.json
glvnd_check
libGLX_installcheck.so.0

So perhaps only just because the builder is able to detect the directory, it just moves forward to building the GPU extensions as expected. In my case libGL.so or other libraries are installed into /lib/x86_64-linux-gnu: but this directory is usually discoverable by ld without any further configurations (see ldconfig -p). To my knowledge, in some other linux systems may have libGL.so, libcuda.so in a /usr/lib/nvidia-XXX directory, depending on how they were installed.

In summary, this PR simply gets rid of a necessity of the hack as in #408 (which would be still good because it can work out-of-box), but if there were some explicit intentions of checking nvidia lib directories, the path to scan might need to be revisted.