Add system install test for pyenv shims

[charliermarsh]

No description provided.

[zanieb]

I started looking into this.... we probably want to avoid building from source each time but pyenv does not support linking a system Python version without a plugin (ref pyenv/pyenv#1244).

We might want to:

1. Create a docker image with some pyenv Python versions prebuilt
2. Publish to GitHub's image registry (can we easily do this in this repository?)
3. Use that in the job
4. Add an optional assertion to our system check script that a given Python version is being used