Automate adding manifest-ci-check label to manifest update PR

[zelinh]

Is your feature request related to a problem? Please describe

Since we enhanced the manifest CI checks by only checking the modified version of manifest instead of all of them, we have to manually add manifest-ci-check label in order to start the GHA to run manifest checks. e.g. #4843

We should automate this process in case sometimes people forget to add such label so no CI checks would be run on modified manifests.

Describe the solution you'd like

When any of the PR modifies files from manifests folder in our build repo, an automatic manifest-ci-check label will be added to the PR and trigger the CI.

Describe alternatives you've considered

No response

Additional context

No response