Fix Windows build.

In dfb52c3d the default language was set to Haskell2010 - by default,
GHC is less strict about the layout rule (controlled by
-XNonincreasingIndentation), but not when we explicitly set the language
to H2010. It turns out we relied on this behavior in the Windows build.

Thanks to Reid Barton for pointing this out.

Signed-off-by: Austin Seipp <austin@well-typed.com>