.github/workflows/backend-test.yml: Fix backend coverage comment

[illume]

It used to be that a coverage report would be sent as a comment to
the PR if the PR has changes in backend/ folder.

Testing

• Try adding a backend/ folder commit, and see the PR comment come

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[illume]

The main issue is that forked PRs don't get access to a secret which gives them permission to comment on the PR.
Because none of our PRs use our own branches any more, it's not commenting anymore.

A possible fix?

Two step workflow:

• backend-test.yml, runs tests, uploads artefacts
• post-coverage-comment.yml, is triggered on workflow_run.completed of backend test finishes, downloads artefacts from the github.event.workflow_run.id and posts the comment (because it has more privs this is allowed)

Still not running code with secrets, the stuff requiring privs is done in a separate workflow. It can download from the forked artefacts because triggered jobs are given a run id.