worker: bump core version

We now build against core commmit
6fa14a8d87b625cf90204a79a415e85c87acc465 (0.0.0.r1468.g6fa14a8d8) [1].

This is a commit after new normalization was adopted. Not *that* much
of the API was broken. Notable changes are that core now returns
core::type_details instead of a std::string. Also the
std::__cxx11::basic_string types are gone and replaced with a simple
std::string.

[1] automata-library@6fa14a8d