[libkworkspace] Fix if getCurrentSeat needs to fallback to old approach
The test for whether "/auto" is supported was bogus.
QDBusAbstractInterface::isValid effectively just confirms the service exists and the path is a legally valid name, not that it has any interfaces there.
This means the fallback path is not run appropriately.
Not tested on a system with out systemd, tested by changing "/auto" to "/asdfasdf" in the line above and confirming if the test behaves correctly.