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]: https://github.com/phusion/digest-sha3-ruby/pull/8