ci: Version of cabal-install installed by setup-haskell should be specified precisely

[mpickering]

At the moment the latest version of cabal-install is installed for validate jobs.

       - name: Install primary compiler                                               
         uses: haskell-actions/setup@v2                                          
         id: setup-haskell                                                       
         with:                                                                   
           ghc-version: ${{ env.GHC_FOR_RELEASE }}                               
           cabal-version: latest 

This had the effect of completely breaking CI when the definition of "latest" changed. It would be better to explicitly pin a specific version of cabal-install here and update it in a controlled manner.

See #10172

[ulysses4ever]

We went back and forth about it, see #8133 The reason is that we didn't quite understand (I think) the issue with cabal-testsuite's custom setup and in particular the versions of Cabal(-syntax) it requires. After reading your message I realized that the comment in the cabal file:

cabal/cabal-testsuite/cabal-testsuite.cabal

Lines 146 to 152 in 1837262

	custom-setup
	-- we only depend on even stable releases of lib:Cabal
	-- and due to Custom complexity and ConstraintSetupCabalMaxVersion
	-- it has to be the latest release version plus
	-- you have to use the latest cabal-install release
	setup-depends: Cabal == 3.12.*,
	Cabal-syntax == 3.12.*,

is not quite precise: it claims that you have to use the latest versions of the library and the latest version of the tool for everything to work, whereas it appears to work with any version provided that it's the same version of the library and the tool. I just confirmed it by checking out master, reverting #10172 and reverting my local cabal-instlall to 3.10.

I used to argue for using latest in CI but after realizing the above, I'd support switching to explicit versioning as you propose. As for updating the version, perhaps the most natural would be to do it as part of the release process. So, whoever pursues it should also update the release wiki to say that after the release of the tool, we should both update the bound in cabal-testsuite custom-setup and update the version of the tool in CI.