Installing GHC prerelease prints Errors but ultimately succeeds

[ulysses4ever]

I noticed that the action (maybe due to how it uses ghcup) spits some errors before succeeding with setting up a GHC prerelease. E.g. (ephemeral link, so I attach screenshot too) here: https://github.com/haskell/cabal/actions/runs/8782600394/job/24097012831

While not the end of the world, maybe something can be changed on the action or ghcup side so that the user doesn't see any errors. I think it'd improve UX.

[andreasabel]

Well, knock yourself off... :-D
Personally, I am keeping the changes to this action minimal (Never change a running system...), but if you are having a lot of time to spare, feel free to experiment...