haskell: servant: Don't hardcode $doc.

Until now this broke on `enableSeparateDocOutput = false`,
as then `$doc` was not defined so it tried to copy stuff to
`/share/doc/servant`.

Fixes regression introduced in commit be63b1994d.

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

@@ -765,7 +765,7 @@ self: super: {
       };
     in overrideCabal super.servant (old: {
       postInstall = old.postInstall or "" + ''
-        ln -s ${docs} $doc/share/doc/servant
+        ln -s ${docs} ''${!outputDoc}/share/doc/servant
       '';
     });