With a recent hackage update, turtle stopped compiling on ghc94. This
commit changes the tests.haskell.incremental test to use the temporary
package instead of turtle.
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.
