check compatibility with GHC prereleases

[geekosaur]

This unnecessarily runs even when a prerelease isn't active, but if it fails then then we have bigger problems (like we do right now with a buggy 3.14.1.0 released).

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[geekosaur]

I filed haskell-actions/setup#104 over the complete inability to get a path or actual version specific to the latest-prerelease install. (I suspect they'll reject it, since it's ghcup-specific but they also use Chocolatey and (for some reason) hvr's ancient unmaintained PPA.)