Add logic to prevent automated PRs from stacking

[william-msft]

• The purpose of this PR is explained in this message or in an issue. If an issue please include a reference as:
  • This fixes Automation PRs from stacking. We noticed this happening because there was a discrepancy where a file like supportedPlatformVersions.md is not up to date, which tricks Automation into thinking there are new updates.
  • The solution is to only check build/constants.yaml for changes, instead of checking the Oryx repo for changes, before running Automation and submitting a PR.
    • Example of stacking:
    • 
  • https://devdiv.visualstudio.com/DevDiv/_boards/board/t/Oryx/Stories?System.AssignedTo=williamhe%40microsoft.com
• Tests are included and/or updated for code changes.
  • In this test no stacking occurred, since constants.yaml had no changes.
  • This is the exit code we check when build/constants.yaml does not and does have a chance. Note the exit code is 0 and 1
    • 
• Proper license headers are included in each file.

[cormacpayne]