hugs: fix meta.mainProgram

this allos lib.getExe to work as expected, e.g. in uses of `nix run ...`
(currently it fails since the binary is `hugs`, but the package `hugs98`)

pkgs/development/interpreters/hugs/default.nix

···
 
   meta = with lib; {
     broken = stdenv.isDarwin;
+    mainProgram = "hugs";
     homepage = "https://www.haskell.org/hugs";
     description = "Haskell interpreter";
     maintainers = with maintainers; [ joachifm ];