CI: Base URLs in azuredeploy.json and README.md should be checked

[hosungsmsft]

A PR from a different branch or from a different fork shouldn't update the base URLs in azuredeploy.json and README.md (the ones with https://raw.githubusercontent.com/Azure/Moodle/master). We should add a step in CI to make sure these values are not changed and reject the PR if any of those are changed.