pipewire: Ensure roundtrips have a timeout

This ensures the portal itself can still start successfully (albeit with
reduced abilities) if PipeWire itself is non-functional.

The most recent case I am aware of where this caused problems was a
friend's system that was upgraded to F35 but had leftover, old PipeWire
configuration files, causing PipeWire's startup to fail and thus
xdg-desktop-portal's startup timeout.