Fix SteamOSBackend isValid check.
Since QDBusInterface isValid is not a good test of whether a dbus service exists or not (according to QDBusAbstractInterface isValid documentation) check if the service file is installed on the disk instead. Fixes spurious SteamOSBackend telling Discover that it is invalid and missing updates, etc.
I know I've got horrible timing. Since Plasma 5.27.10 is tomorrow. Is there going to be a .11? Either way I'll be rebasing these 2 fixes on master for the Plasma 6/master branch next.