fix broken upload_to_zoltar.yml action

[Matthew Cornell]

"Trigger zoltar upload" actions (upload_to_zoltar.yml) are failing due to 1) GitHub Actions now using Python v3.12 as the default, which removed the imp module that our https://github.com/reichlab/pykwalify/ fork uses. (Details in this Slackthread).

@nickreich suggests:

if it’s “easy” to fix the python version that we are using via that setup-python action then I’d vote for trying that and seeing if we can continue to limp along here without needing to refactor the current zoltar uploads code.

I found https://github.com/actions/setup-python , which might do the trick. We'd specify a pre-3.12 Python version (3.11 I suppose).