Citylink "Airlink" bus. Something like £15 one way, £22ish return (lasts for a month).
It normally leaves stance C at Edinburgh, but the normal bus stops are out of action. You need to go head left towards the tram stop, where you'll see the new bus stances being used now. No idea of this is to do with Covid or renovations.
Glasgow bus leaves from stance 5, though it's unmarked.