[WIP] Fix ref on GitHub Actions checkout step

[mrcasals]

🎩 What? Why?

We've noticed more funky issues with coverage reports. More precisely, the issue seems to come from a bunch of factors, but let me try to explain the issue first.

How to reproduce

1. Create a PR
2. Make one of the workflows fail for random issues apart from RSpec. The best way to reprocue it is getting a random failure while trying to install the Ruby dependencies.
3. Retry that workflow, see it passing successfully.
4. Open the coverage reports. See that there's no reports for the files in the first-failed-then-successful workflow
5. Open that workflow on GitHub Actions, open the coverage report step and see it failing.

To be fair, the step 2 should be fixed by #5910, but the real issue persists.

Reasons why it's failing

If you check the checkout step on a successful workflow, you'll see it's creating a special branch names pull/<PR id>/merge (replacing <PR id> with the correct number). This creates a new commit.

If you check the same step for a first-failed-then-successful workflow you'll see it's not using that werid branch name, but instead it's using the real branch name. This inconsistency makes the coverage report to fail, for some reason.

Checking the actions/checkout repository, I don't see any specific place where that weird branch name is specified, so this would only mean it comes from the GitHub Actions environment itself.

📌 Related Issues

• Related to #?
• Fixes #?

📋 Subtasks

• Add CHANGELOG entry
• Add documentation regarding the feature
• Add/modify seeds
• Add tests
• Another subtask

📷 Screenshots (optional)

[mrcasals]

Closing in favor of #5991