This allows packagePlatforms to pick up on the overall supported
platforms and schedule builds on Hydra for more than the evaluation
platform (usually x86_64-linux).
Since the rust writer doesn't seem to get fixed on darwin, we'll just
wrap the haskell writer test in our own derivation (which is possible
since tests.writers exposes a bunch of internals via passthru) and
expose it via tests.haskell which are already in mergeable.
Finally a way to test the (hopefully) working haskell writer on darwin
again!
Contrary to database-id-class, linear is part of stackage and actively
maintained, so the test is less likely to fail due to version
constraint issues as it is currently.
