GCC compile error with Actor

[snej]

GCC 12 in GitHub CI is failing to compile Actors. It's trying to use the deleted default constructor instead of the customized ones that take the coroutine method's signature.

/home/runner/work/crouton/crouton/tests/tests.cc: In member function ‘crouton::Future<long int> TestActor::fibonacciSum(int) const’:
/home/runner/work/crouton/crouton/tests/tests.cc:142:5: error: use of deleted function ‘crouton::ActorMethodImpl<T>::ActorMethodImpl() [with T = long int]’
  142 |     }
      |     ^
In file included from /home/runner/work/crouton/crouton/include/Crouton.hh:20,
                 from /home/runner/work/crouton/crouton/tests/tests.hh:19,
                 from /home/runner/work/crouton/crouton/tests/tests.cc:19:
/home/runner/work/crouton/crouton/include/Actor.hh:129:9: note: declared here
  129 |         ActorMethodImpl() = delete;
      |         ^~~~~~~~~~~~~~~