fix(ui): ensure prelude gets printed before ui starts (#8265)
### Description
We had a race previously between switching to the alternative screen and
the prelude being run. This PR moves the prelude printing to always
happen before we switch to the alternative screen.
We also now flush `stdout` to ensure all of the prelude has been written
before switching to the alternative screen.
### Testing Instructions
No longer have bleed in `Terminal.app`
<img width="1357" alt="Screenshot 2024-05-31 at 10 52 23 AM"
src="https://github.com/vercel/turbo/assets/4131117/6d0d0706-e78c-4847-bdf5-0e6f81d71ca3">
