enable check

DocTest errors
```
[ERROR  ] [ThreadId 337] Could not import module: Distribution.Server.Features.UserNotify. This can be caused by a
number of issues:

[ERROR  ] [ThreadId 339]     Could not load module ‘Distribution.Server.Util.Markdown’.

[ERROR  ] [ThreadId 338]     Could not load module ‘Distribution.Server.Util.Validators.Internal’.
```

flake.nix

@@ -21,7 +21,7 @@
  haskellProjects.default = {
  basePackages = pkgs.haskell.packages.ghc98;
  settings = {
- hackage-server.check = false;
+ # hackage-server.check = false;
  # https://community.flake.parts/haskell-flake/dependency#nixpkgs
  tar = { super, ... }:
  { custom = _: super.tar_0_6_2_0; };