Replace pavucontrol w/ pipewire-native pwvucontrol

Hi, @mightyiam! I was reviewing your flake, looking for enlightenment, and I saw that you were still using the pulseaudio volume control app, instead of the pipewire-native one¹ that has been available for some time now. So, in case this was not intentional, here's my iota of a contribution :stuck_out_tongue_winking_eye:. Didn't know about `qpwgraph`, which I have now adopted, thanks for that! Happy hacking! ¹: See https://github.com/saivert/pwvucontrol