Push runs skip plugin checks

[Monti-27]

The plugin workflow runs after pushes to main, but it compares HEAD with origin/main. Both refs point to the pushed commit, so every plugin job is skipped.

The latest main run skipped the kmesh build even though the merge changed that plugin: workflow run.

PR runs are not affected. The push comparison should use the previous commit from the push event.

[rajeshsingh241]

Hi @Monti-27 , thanks for the clear writeup! I'd like to take this on.
Looking at it — on a push event, origin/main/HEAD both already point to the newly pushed commit by the time the workflow runs, so any diff between them is always empty. Using github.event.before (the commit before the push) as the diff base instead should fix it, with a fallback for the all-zero SHA case on force-pushes or new branches. Can I go ahead and open a PR for this?