haskell-hpack: fix override for latest version

pkgs/development/haskell-modules/configuration-common.nix

···
   # Newer hpack's needs newer HUnit, but we cannot easily override the version
   # used in the build, so we take the easy way out and disable the test suite.
   hpack_0_20_0 = dontCheck super.hpack_0_20_0;
-  hpack_0_21_0 = dontCheck super.hpack_0_21_0;
+  hpack_0_21_2 = dontCheck super.hpack_0_21_2;
 
   # Stack 1.6.1 needs newer versions than LTS-9 provides.
   stack = super.stack.overrideScope (self: super: {