refactor: remove wheel index branching from test scripts

[gforsyth]

Looks like at least one test job has the pytorch index set to nightlies:

if [[ "${CUDA_MAJOR}" == "12" ]]; then
  PYTORCH_INDEX="https://download.pytorch.org/whl/cu126"
else
  PYTORCH_INDEX="https://download.pytorch.org/whl/nightly/cu130"
fi

that's in test_wheel_cugraph-pyg.sh

Thanks for the fix. Is there any way we can remove that from the wheel scripts? Would be good to reduce technical debt here.

Originally posted by @alexbarghi-nv in #355 (comment)