[collab] Improvements for COLLAB testing support

[mmcky]

There are currently three lectures that install gpu related software needed to be installed for collab compatibility testing.

They have header code-cells such as

:tags: [hide-output]
!pip install numpyro jax

following the included gpu admonition

While these are not being issued with upgrade so they won't replace the gpu software installed by the GitHub Action scripts, ideally these would be set to skip-execution so they don't execute at all.

Perhaps a context based execution would be useful feature for mystmd