Consider dropping GHA runs against pushes

[brettcannon]

Since we require PRs to be synced to 'main', does running on push actually give us any benefit? As long as we don't cheat with admin privileges, I would think a successful run for a PR will be exactly the same as one on push. If that logic holds, then we are basically wasting free credits and energy running on push.

If people agree we can add logic such that runs against pushes only occur against other repos (i.e. people's personal forks); basically we add an if guard on the jobs.

[di]

The one benefit I see here is that folks who push branches to the PyPA remote will see CI status on their commits prior to making a pull request. Perhaps I'm the only one that does this, though. If so, I could just push branches to my fork instead.

[brettcannon]

I personally push to my fork for this, but then I might be the only one. 😄

[pradyunsg]

I push to my fork too!