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] c7861b85c4
[2] 8a0a5ec3a3

nixos/tests/hydra/create-trivial-project.sh

@@ -44,6 +44,8 @@ cat >data.json <<EOF
   "enabled": "1",
   "visible": "1",
   "keepnr": "1",
+  "enableemail": true,
+  "emailoverride": "hydra@localhost",
   "nixexprinput": "trivial",
   "nixexprpath": "trivial.nix",
   "inputs": {