Warn PR author when eng/common is modified in consumer repo

In arcade-powered repos, eng/common is logically a (readonly) mirror directory, which dotnet-maestro bot dutifully updates.

If a PR is submitted by a human being that updates a file under this directory, it is easy to misremember during the PR review. That causes a merge conflict in the next arcade update and mostly results in reverting the manual changes made earlier.

It would be nice if a bot post a comment to such PRs to help reminding reviewers about readonly-ness of eng/common, something to the effect:

> Reminder: `eng/common` is a mirror directory. Please submit changes in dotnet/arcade repo to avoid conflicts during the subsequent arcade update.

ps: such reverts took place in dotnet/runtime repo few times, and currently eng/common in aspnetcore repo is diverged.