ci: build every pull request, not only those based on main

[rasros]

Summary

build.yml runs on every pull request rather than only those based on main.

Why

The workflow already ran on pull requests, so this is not about turning CI on; it is about which
pull requests it covers. pull_request: branches: [ main ] means a stacked pull request — one branch
based on another, targeting that other branch — gets no checks at all and can merge unverified.

klause hit this and dropped the filter; the change here is the same, comment included, so the repos
agree. push stays restricted to main, so this adds no duplicate runs beyond the stacked case it
is meant to cover.

Testing

The workflow parses, and its trigger now matches klause's byte for byte: pull_request present with
no base filter, push still [ main ].