They can't because they'd have to stop printing money
Instead they'll print money to acquire Bitcoin as much as they can until people stop accepting freely printed money
Discussion
Why they would have to stop printing money? A CBDC may be programmed to have a circulating supply that can be altered (in order to emulate the printing of money)
As long as the supply of dollars increases and the supply of Bitcoin doesn't, demand will also increasingly shift to Bitcoin until there is no demand for dollars anymore