Fix CLA check for first-time contributors

[griggt]

Now that Github Actions requires maintainer approval for workflow runs on PRs from first-time contributors, the CLA check is performed on the wrong user id (workflow approver rather than PR author).

A recent example of this is PR #12718, seen here: https://github.com/lampepfl/dotty/runs/2764331890?check_suite_focus=true#step:3:4

[sjrd]

According to https://github.community/t/how-to-get-the-author-of-a-pr/135089, the event type will be different based on what triggers the issue, and hence the way to access the login is different. How can we be sure that this will work in (at least) the following cases:

• Existing contributor opens pull request
• New contributor opens pull request, gets approved by maintainer
• Pull request author (force-)pushes a new commit
• Someone else (force-)pushes a new commit
• Maintainer relaunches a build manually for their own PR, or for someone else's PR

?

[griggt]

How can we be sure that this will work in (at least) the following cases:

* Existing contributor opens pull request

* New contributor opens pull request, gets approved by maintainer

* Pull request author (force-)pushes a new commit

* Someone _else_ (force-)pushes a new commit

* Maintainer relaunches a build manually for their own PR, or for someone else's PR

I informally tested most of these situations by inspection over here: https://github.com/griggt/wf-by-branch/actions
which for example demonstrated that ${{ github.event.sender.login }} is a poor choice because it fails in the fourth case.

I can clean that effort up if you like and ensure it covers the cases above.

I'd certainly welcome any suggestions on how to achieve proper test coverage here. Unfortunately the official documentation seems to be somewhat lacking.

[griggt]

the event type will be different based on what triggers the issue

Also of note, the only event type which triggers the cla workflow is pull_request:

https://github.com/lampepfl/dotty/blob/fdbb94f29b05fb712c04b981f462ae65b447fc39/.github/workflows/cla.yml#L1-L3

[sjrd]

the event type will be different based on what triggers the issue

Also of note, the only event type which triggers the cla workflow is pull_request:

Right. Then perhaps github.event.pull_request.user.login is indeed always set up, and hopefully it contains the right thing. I guess we can try it out...