dgit-repos-server: t2u: Implement CCing the tag signing key's owner
This fixes #23 (closed) (the remainder thereof).
The priority sort code is not really tested. I think doing so requires adding a new UID to the Senatus test key. But gpg on bookworm can't edit secring.gpg files. So we could (i) edit secring.gpg in a chroot running older Debian (ii) migrate our test suite to use private-keys-v1.d (iii) migrate our test suite to use sequoia pgp (iv) decide this is not worth doing for now as the most important cases are tested.