Add `cabal check` to `checks`

In arion I had a separate build with

                  checked = super.haskell.lib.overrideCabal pkg (o: {
                    postConfigure = ''${o.postConfigure or ""}
                      if ! ${hsuper.cabal-install}/bin/cabal check;
                      then
                        echo 1>&2 ERROR: cabal file is invalid. Above warnings were errors.
                        exit 1
                      fi
                    '';
                  });

I think we could touch the outputs and exit 0 to speed this up.
Doing it in runCommand would be fragile.
This should probably call Setup.hs instead of cabal-install.

Running it as part of the main build isn't ideal because the check may be more strict than necessary. I think it should only run in CI.

Start by reading how the existing checks output is defined and invoked in CI, then compare the proposed Setup.hs path with the mentioned cabal-install and runCommand approaches. Done means cabal check runs only in CI, reports invalid Cabal files as failures, and does not slow the main build.