fix: ext_name is used in the directory structure

The service name is taken from the file e.g. event_store.service so if
ext_name has hyphens these won't match up and we'll get errors like

LifecycleEvent - AfterInstall
Script - bin/deploy-extract-release
[stdout]==> Removing previous release /srv/event-store/current
[stdout]==> Extracting release from tar file /opt/codedeploy-agent/deployment-root/5b9577ac-428f-4dbf-82ce-79d9fe70d88a/d-7N1LS0I86/deployment-archive/bin/../event_store.tar.gz
Script - bin/deploy-set-perms
Script - bin/deploy-enable
[stdout]==> Enabling systemd unit event_store
[stderr]Failed to enable unit: Unit file event_store.service does not exist.

lib/mix/tasks/deploy.ex

@@ -25,8 +25,7 @@ defmodule Mix.Tasks.Deploy do
 
     service_name = app_name |> to_string()
 
-    ext_name =
-      app_name
+    ext_name = app_name
       |> to_string
       |> String.replace("_", "-")