Currently that’s not possible, though it might be with github actions, it’s not something that we natively support. One option that you have is to filter your webhooks using a lambda function. Have github send the webhook to the function, then in the function you can parse the payload and then decide whether or not to trigger the build from there.
You can find the code here.
I’ve added the instructions in the README.
Notice that you’ll also need to contact Netlify’s support and request the disabling of automatic deploys for all your sites in the mono-repository, while still maintaining the repo linked to Netlify, as mentioned here.