Add timeout to conda-python-tests job

There were two instances recently (below) where some Python test errors caused the `conda-python-tests` job to run/hang for ~4 hours.

- rapidsai#981 (comment)
- rapidsai#1081 (comment)

To prevent this from happening again in the future, I've added a reasonable timeout of 45 minutes to that particular job.

The job usually takes ~25 minutes to complete, so 45 minutes should be plenty.

This timeout will help prevent jobs from hanging and thus help preserve our finite GPU capacity for CI (particularly for `arm` nodes).

.github/workflows/pr.yaml

@@ -28,6 +28,7 @@ jobs:
     with:
       build_type: pull-request
   conda-python-tests:
+    timeout-minutes: 45
     needs: conda-python-build
     secrets: inherit
     # TODO: Switch this testing branch to "cuda-118" after `cudf` `3.10` builds are out.