Check for sdist integrity in CI

[tdammers]

As mentioned in https://github.com/input-output-hk/cardano-base/pull/365, some packaging problems currently go undetected in CI.

Proposed fix: add an "sdist" step to the Haskell workflow, which runs cabal sdist all - this command will print any problems it finds, and exit with a nonzero status if there were any.