Pin Python version in plbase image

[nwalters512]

Currently, we always install the latest version of Minoforge when building the prairielearn/plbase image:

https://github.com/PrairieLearn/PrairieLearn/blob/310ff716139f576b20280456a64b3b5fb34f5a93/images/plbase/plbase-install.sh#L61

However, this has the unintended side effect of also installing the latest version of Python. We should update that so that it pins Python to a known version, which will give us control over how and when a new version of Python gets consumed.

This may look either like pinning Miniforge itself, or it might be possible to instruct Miniforge to install a specific version of Python instead of the latest.

See also #6447.

[eliotwrobson]

Probably worth handling with https://github.com/PrairieLearn/PrairieLearn/issues/6536, since the version can be pinned to 3.11. This might need to wait until 3.11 is available as part of Miniconda by default, but it's already available as a downloadable package in conda-forge: https://anaconda.org/conda-forge/python

[SethPoulsen]

duplicate? https://github.com/PrairieLearn/PrairieLearn/issues/6447

[reteps]

Closed via https://github.com/PrairieLearn/PrairieLearn/pull/10769