Only build static executable in top-level niv attribute

This reduces the closure size by about 2G.

default.nix

@@ -46,6 +46,7 @@ with rec
   haskellPackages = pkgs.haskellPackages.override {
     overrides = _: haskellPackages: {
       niv =
+        pkgs.haskell.lib.justStaticExecutables (
           pkgs.haskell.lib.failOnAllWarnings (
             pkgs.haskell.lib.disableExecutableProfiling (
               pkgs.haskell.lib.disableLibraryProfiling (
@@ -54,6 +55,7 @@ with rec
                 )
               )
             )
+          )
         );
     };
   };