Enable github CI for all (including external) PRs

[soenkehahn]

Currently, our github CI doesn't run for PRs from forked repos, e.g. for this one:
#349

This hopefully fixes that.

I think we still have to manually approve CI runs from external forks. But that seems sensible for now.

Update: I first tried out on: [push, pull_request], but that means that for internal PRs we always get duplicated runs, since they're both a PR and a push to a local branch. I changed the config to now:

• Enable CI only for pushes to the main branch, and
• enable CI for all PRs.

But this means that we won't get CI runs for pushes to branches other then main in this repo, if there's no PR open for that branch. So if you want a CI run, you'd have to open a PR (possibly a draft PR). I think this is acceptable. WDYT?

[sol]

Currently, our github CI doesn't run for PRs from forked repos, e.g. for this one: #349

This hopefully fixes that.

I think we still have to manually approve CI runs from external forks. But that seems sensible for now.

Update: I first tried out on: [push, pull_request], but that means that for internal PRs we always get duplicated runs, since they're both a PR and a push to a local branch. I changed the config to now:

• Enable CI only for pushes to the main branch, and
• enable CI for all PRs.

But this means that we won't get CI runs for pushes to branches other then main in this repo, if there's no PR open for that branch. So if you want a CI run, you'd have to open a PR (possibly a draft PR). I think this is acceptable. WDYT?

That's basically what I have been doing for most of my repositories for quite a while now, and it works very well for me.

https://github.com/sol/hpack/blob/main/.github/workflows/build.yml#L3-L15

You may also want that first three lines, so that in-progress builds are canceled when you add additional commits to a PR.