How to trigger amplify build only for certain folder change

Question

I have a MONO repo structure. Something like this:

  backend
  frontend1 (react app) -> this contains amplify.yml
  frontend2 (react app) -> this contains amplify.yml

I have 2 frontend apps both deployed on amplify. I also have PR previews enabled on both the apps. Now every time I open a PR - both frontend1 & frontend2 are building. This is NOT ideal because it's $$ expensive & makes things slow.

Is there a way to skip folders? or mention trigger folders in amplify.yml?

I want to do this in amplify.yml only, not with another external mechanism.