fix: datetime.datetime.utcfromtimestamp deprecation warning

[ianna]

Fixes issue #3337

[jpivarski]

Thanks for updating utcfromtimestamp, for keeping ahead of deprecations. I'm not sure why the environment variable SOURCE_DATE_EPOCH (not defined in my terminal...) gets precedence over time.time(), which I think should always work. If this were in the main codebase, I'd be more concerned because it would be introducing a dependence on the environment variable state, rather than anything in Python, but I think it's fine for the developer scripts.

The failing tests are all coming from JAX. I started running a test on the main branch to see if it's something that changed in JAX: https://github.com/scikit-hep/awkward/actions/runs/12259902669 All of these test were passing just a few days ago, so it might be due to a new version of JAX.