CCM-1876: revert eventpub source change

Author: alexnuttall

infrastructure/terraform/components/sandbox/module_eventpub.tf

@@ -1,5 +1,5 @@
 module "eventpub" {
-  source = "https://github.com/NHSDigital/nhs-notify-shared-modules/releases/download/v2.0.20/terraform-eventpub.zip"
+  source = "git::https://github.com/NHSDigital/nhs-notify-shared-modules.git//infrastructure/modules/eventpub?ref=v2.0.19"
 
   name = "eventpub"