De-dupe action triggers

[plocket]

Implement this code:

jobs:
  job_name:
    if: (github.event_name != 'pull_request' && ! github.event.pull_request.head.repo.fork) || (github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork)

https://github.com/IHaskell/IHaskell/blob/60a750f5b038a17717609d8a40be0fafd69e1c35/.github/workflows/stack.yml#L37

It could make a good example [for our users]. Is there a reason to test the event happening at the same time? There are plenty of non-pr times when we push, so we should see plenty of both triggers.

[plocket]

I misrepresented this a bit. To be clear, the only time the PR workflows will run is if the PR is from a fork. We'll have to keep an eye out for when we should test fork PRs.