.github/workflows: use ofborg-eval context for pending status

[ncfavier]

What we want

If I understand correctly, we want new(ly updated) PRs to get a "Waiting for OfBorg" pending status, so that they stay pending at least until OfBorg is summoned.

What is currently happening

When a PR is created, the pending-set.yml workflow creates a pending status with context Waiting for ofborg and description "This pending status will be cleared when ofborg starts eval.".

However, the pending-clear.yml workflow only runs when the OfBorg check suite becomes completed, which happens when all of the check runs (not statuses) created by OfBorg are completed (you can see the check runs posted by OfBorg in the "Checks" tab of a PR, under the "OfBorg" drop-down). This only happens once OfBorg finishes evaluation, because it then posts the "Evaluation Performance Report" check run, but it can happen again e.g. when all queued packages are done building.

As far as I can tell, the ofborg-eval-started status introduced in NixOS/ofborg#527 does not change anything (since it is a status and not a check) and can be removed (independently from this change).

Proposed fix

Instead of adding a pending status with context Wait for ofborg, make the context ofborg-eval and the description "Wait for OfBorg...". That way, the status will be reused by OfBorg when it starts evaluation and we don't need to clear it any more.

Will this work?

I think so. I tested this on my fork. After creating the PR, the pending status was set as expected:

Then, after simulating OfBorg by running

curl -H "Authorization: Bearer $MY_TOKEN" \
    -d '{"context": "ofborg-eval", "state": "success", "description": "^.^"}' \
    "https://api.github.com/repos/ncfavier/nixpkgs/commits/$SHA/statuses"

the status was correctly overwritten:

[zowoq]

Let's see if it works.

[ncfavier]

One small problem I did not anticipate: ofborg is so fast that there's a race condition where the "Waiting for OfBorg" message is written after it starts. Not too bad since it will be overwritten again soon, but still annoying.