Add aggregator build as github validation step

[iloveeclipse]

See comment from @laeubi on eclipse-platform/eclipse.platform.releng.aggregator#1750 (comment)

Tycho uses a (quick) build of the aggregator 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.

[laeubi]

I have now started some tests here:

• Update verify-platform.yml laeubi/eclipse.jdt.core#7

[laeubi]

@iloveeclipse can you suggest a change that will generate a compare error so I can check the action works?

[iloveeclipse]

can you suggest a change that will generate a compare error so I can check the action works?

Revert #1906, it should cause 3 comparator errors in platform