Nothing's stopping you from simply making an attack with Haste and then readying an action though.
The wording from Ready in the PHB makes it sound like you can't take any action if you plan to ready.
Or even take the Ready action first, and your one attack from the Haste action after. Not that it changes anything.You can perform the Haste action first and use your regular action to Ready.
That works fine, but you give up using a SCAG cantrip for the first. Should be fine for the readied haste action.Nothing's stopping you from simply making an attack with Haste and then readying an action though.