Add aggregator build as github validation step

See comment from @laeubi on https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/issues/1750#issuecomment-1911818888
> Tycho uses a [(quick) build of the aggregator](https://github.com/eclipse-tycho/tycho/actions/workflows/verify-platform.yml) to catch > issues early, maybe JDT can use similar to be aware of such unanticipated changes when compiler is fixed, but its just an idea, total > runtime is about 17 minutes.

The idea is: new github action could be added to JDT PR validation workflow at github. The action would run SDK aggregator build with latest ecj state from PR. This would allow contributors see if the change in compiler "breaks" SDK build before actual nightly SDK build fails.

Contributions are welcome.