nixos/hydra: incorporate upstream changes and update test

During the last update, `hydra-notify` was rewritten as a daemon which
listens to postgresql notifications for each build[1]. The module
uses the `hydra-notify.service` unit from upstream's Hydra module and
the VM test ensures that email notifications are sent properly.

Also updated `hydra-init.service` to install `pg_trgm` on a local
database if needed[2].

[1] https://github.com/NixOS/hydra/commit/c7861b85c4c3cc974b27147bbf3cc258b9fe9cc3
[2] https://github.com/NixOS/hydra/commit/8a0a5ec3a3200d4f4d4d38f87d0afdb49f092b39