ci: stop requesting the runner's preinstalled cabal

[jimmykirk]

Unverified — please read the caveat at the bottom before merging.

Problem

CI has failed on every run since the merge of #1, including runs on main:

failure   diagram-focus-search            (#8)
failure   main                            (merge of #7)
failure   fix-coslice-output-sensitive    (#7)
failure   main                            (merge of #5)
failure   fix-coslice-crossproduct        (#5)
success   main                            (merge of #1)

It dies in Set up GHC, before anything is compiled, so it is unrelated to
any of the PRs it is failing on:

[Error] The version '3.12.1.0' of the tool cabal is not installed.
Install error was: cabal-3.12.1.0 is already installed; if you really want to
                   reinstall it, you may want to run 'ghcup install cabal --force 3.12.1.0'
Set error was: The version '3.12.1.0' of the tool cabal is not installed.
##[error]All install methods for cabal 3.12.1.0 failed

ghcup contradicts itself: ghcup whereis cabal 3.12.1.0 returns
/usr/local/.ghcup/cabal/3.12.1.0/cabal, while ghcup set reports the same
version as not installed, and the install path then declines because it is
installed. That is the runner image's preinstalled cabal carrying ghcup
metadata that no longer matches — a runner image update, not anything in this
repository.

cabal-version: "3.12" resolves precisely onto that copy.

Change

Ask for latest instead, so setup takes the clean install path rather than
trying to adopt the image's. ghc-version stays exactly pinned at 9.10.2.

Caveat — I could not test this

The failure only reproduces on GitHub's runners, and workflows are disabled on
forks until enabled by hand, so I have no way to prove the fix from here. The
diagnosis is solid (the log is unambiguous about what fails and why); the
remedy is the standard one for this class of ghcup state mismatch, but it is
reasoning rather than evidence.

If you would rather not merge an untested CI change — entirely reasonable —
the alternatives are pinning an exact newer version instead of latest, or
adding ghcup install cabal --force ahead of the setup step. Happy to switch
to either.

I am also happy to verify it properly if you enable Actions on my fork's
mirror of this branch, or you can push the same one-line change to a branch
here and watch it directly.