Add libGL to image dependencies.

[bdice]

Potential fix for OpenCV issues in rapidsai/cucim#471.

(There are alternative fixes to consider, so this PR may be closed if another route is taken.)

[bdice]

We are adopting another solution here: rapidsai/cucim@6045729

[jakirkham]

Is there more context on why this is being closed and that option being chosen?

[bdice]

@jakirkham I discussed on a call with @ajschmidt8. We decided to retain the same testing method that cucim used previously (pip install opencv-python-headless) because it was working in Jenkins (gpuCI).

To use conda packages of opencv (which require libGL), we'd have to install libGL into our CI images (which was not present in the old CI images either). We'd like to avoid adding specific apt/yum dependencies to the base images unless there is no way around it. In this case, there is a status quo solution (use the "headless" pip package) which retains cucim's existing testing strategy and doesn't change our CI images.

There are a couple discussions about creating "headless" opencv conda-forge packages that do not depend on libGL, which we would benefit from having:

• Headless version??? conda-forge/opencv-feedstock#293
• ImportError: libGL.so.1: cannot open shared object file conda-forge/opencv-feedstock#321

"Headless" versions of opencv already exist on other channels such as fastai.