community/pipewire: kill previous instance before launch

Patchset from Pranjal Kole pranjal.kole7@gmail.com via aports #3959:

/usr/bin/pipewire was not being killed previously.

Additionally, the order in which the processes are killed has been
changed to avoid this error message from pipewire-media-session:

[W][77952.698669] ms.core      | [ media-session.c: 2211 core_error()] error id:0 seq:799 res:-32 (Broken pipe): connection error