Downgrade to cabal-install-3.12

[phadej]

I stumbled on few bugs in cabal-install-3.14

• [3.14] Initial configuration of programs does not use build-tool-depends haskell/cabal#10692
• cabal test doesn't change to package directory haskell/cabal#10704

Especially the latter affects haskell-ci in a blocking way. So if the issues are not addressed by 2025-01-12, I'll downgrade the cabal-install used. This may result with subpar experience with GHC-9.12, but less so in CI setups.