haskellPackages.candid: add workaround for doctest failure

This is fixed properly in candid 0.3 which we'll have soon, but in
the meantime we can just disable the offending warning. A real shame
that doctests are influenced by compiler warnings.

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

···
   hw-xml = assert pkgs.lib.versionOlder self.generic-lens.version "2.2.0.0";
     doJailbreak super.hw-xml;
 
+  # doctests fail due to deprecation warnings
+  candid = assert pkgs.lib.versionOlder super.candid.version "0.3";
+    appendConfigureFlags super.candid [ "--ghc-option=-Wno-deprecations" ];
+
 } // import ./configuration-tensorflow.nix {inherit pkgs haskellLib;} self super