ci: fix CI by using pytorch==2.0.1 and torchaudio==2.0.2 explicitly
Last week, just saying "conda install pytorch torchaudio -c pytorch" resolved
to 2.0.1 and 2.0.2, respectively. Today, they resolved to 1.13.1 and 0.13.1,
breaking the build. I don't know why that changed, but forcing these versions
fixes the build so that's good enough for me.
