feat(single-instance): add logging when closing a 2nd instance

[aecsocket]

While debugging a Tauri app which was crashing on startup, I got into a state where every time I opened the app, it would close unexpectedly with exit code 0. I had to open gdb, break exit and use bt to figure out that it was actually Tauri closing the app due to the single-instance plugin. I had a zombie process taking the DBus SingleInstance name for my app.

This PR adds a tracing::info message when exit(0)ing the app on Linux if the single instance DBus name is already taken.