haskell: fix shellFor test

In #296645, envFunc (and thus shellFor) was changed from
`mkDerivation {phases=[..]}` to `runCommand ..`. So since
then, the derivation of shellFor contains a `buildCommand`
("echo $nativeBuildInputs $buildInputs > $out"),
and as a result the custom phases defined in shellFor's
test were ignored.

pkgs/test/haskell/shellFor/default.nix

···
         maintainers = allMaintainers;
         inherit (cabal-install.meta) platforms;
       };
+    # `shellFor` adds a `buildCommand` (via `envFunc -> runCommandCC`), which
+    # overrides custom phases. To ensure this test's phases run, we remove
+    # that `buildCommand` from the derivation.
+    buildCommand = null;
   })