Validate CI should display the output of `cabal-plan`

[andreasabel]

It would be good to see which dependencies are picked up to build cabal-install, so a step in the CI scripts to stream the output of cabal-plan after (ideally before) the build would be great.

Example of such a step: https://github.com/haskell/hackage-server/blob/f6c1e48f8da9293055ff9e86aabd8a33f6594893/.github/workflows/haskell-ci.yml#L192-L195

[jneira]

Maybe it couldbe added to validate.sh and make it ci agnostic. We are already using it there:

cabal/validate.sh

Line 350 in 1003ed6

timed $CABALPLAN --version