Use the full path to retrieve pipewire PID
The change added in 921bb9ab was not enough because other processes ending by "pipewire" are running. So, instead use the full path to really catch the main pipewire process, and for more safety ask pgrep to only return a unique PID for the oldest process.
Tested with apertis/v2024pre sdk, the following ps call doesn't complain anymore but the test fail because the apparmor rules for wireplumber need to be updated.
Edited by Dylan Aïssi