Add nightly runs to the public CI

Author: andystaples

eng/ci/public.yml

@@ -4,6 +4,14 @@ trigger:
     include:
     - dev
 
+schedules:
+- cron: "0 0 * * *"
+  displayName: Daily midnight build
+  branches:
+    include:
+    - dev
+  always: true
+
 resources:
   repositories:
     - repository: 1es