Add hydra-notify to devshell

I came across https://github.com/NixOS/hydra/issues/751 and realized
that hydra-notify is responsible for creating the additional jobsets in
a declarative file. My declarative testing works in dev now.

foreman/start-notify.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# wait for hydra-server to listen
+while ! nc -z localhost 63333; do sleep 1; done
+
+touch .hydra-data/hydra.conf
+HYDRA_CONFIG=$(pwd)/.hydra-data/hydra.conf exec hydra-notify