CI: add CUDA 13 conda python tests

[jameslamb]

Description

Initial CUDA 13 CI was added in #286, but it skipped conda Python tests because there were not yet PyTorch CUDA 13 packages available on conda-forge.

Those tests should be restored once those packages exist.

Benefits of this work

• ensures everything we release is tested, improving release confidence

Acceptance Criteria

Conda Python tests for CUDA 13 are running in CI here.

Approach

Wait until there are pytorch CUDA 13 packages on conda-forge:

• general CUDA 13 tracking: Enable CUDA 13.0 binaries pytorch/pytorch#159779
• PRs will show up at https://github.com/conda-forge/pytorch-cpu-feedstock

Once they're available, revert the workarounds from #286

Notes

N/A

[jameslamb]

I've started a new nightly test run: https://github.com/rapidsai/cugraph-gnn/actions/runs/21642972899

Just to get faster feedback on whether this is truly done or whether follow-up changes are needed for nightlies.

[jameslamb]

I never made it back to check yesterday but anyway.... yes the nightly tests passed, so I think this is really and truly done 😁