netlify: skip build if nothing changed (#5039)

The build now follows this logic: for non-pull-requests, i.e. tag pushes and merges to
main, we'll check if anything has changed in the relevant files, compared to
the previous deployment.

For PRs, we'll check for changes in the files relative to `main`.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>