gem-config: add digest-sha3

`digest-sha3` is a C-extension gem which fails to build on Nix because
it uses non-literals as format strings which is forbidden by the default
Nix hardening settings. There is a pull request to fix that ([1]), but
the gem seems to be abandoned.

This PR disables the "format" hardening for `digest-sha3`.

[1]: phusion/digest-sha3-ruby#8

pkgs/development/ruby-modules/gem-config/default.nix

@@ -94,6 +94,10 @@ in
     '';
   };
 
+  digest-sha3 = attrs: {
+    hardeningDisable = [ "format" ];
+  };
+
   ethon = attrs: {
     dontBuild = false;
     postPatch = ''