Set low priority for scheduled runs (#103)

This PR sets the Priority for scheduled pipeline runs to be low. This is
part of an effort to reduce the daily deadlock that we are facing with
our various pipelines.

It also reschedules these pipeline runs to 6am UTC.

Author: castrodd

eng/ci/official-build.yml

@@ -14,7 +14,7 @@ schedules:
 # Build nightly to catch any new CVEs and report SDL often.
 # We are also required to generated CodeQL reports weekly, so this
 # helps us meet that.
-- cron: "0 0 * * *"
+- cron: "0 6 * * *"
   displayName: Nightly Build
   branches:
     include:
@@ -39,6 +39,9 @@ extends:
             name: 1es-pool-azfunc
             image: 1es-windows-2022
             os: windows
+            ${{ if eq( variables['Build.Reason'], 'Schedule' ) }}:
+              demands:
+              - Priority -equals Low
 
         stages:
             - stage: BuildAndSign